Welcome to the forum. I almost never use the Fix relation. I would recommend deleting the Fix relations and fully defining the triangle with relations and dimensions first. (You'll know that it's fully defined when all of the sketch entities of the triangle turn black. Be sure to define the position of the triangle in relation to the origin.) Then you should be able to use dimensions to place the rectangle where you want it.
Edit: Looking back at your original post, you said that you had used the Fix relation to anchor the triangle. I want to point out that selecting one or two entities of the triangle and applying the Fix relation only fixed those individual entities. In your screenshot it appears that only the horizontal line and it's left end are fixed. Please note that the other lines and line intersections are blue and not black. This indicates that the Fix relation didn't apply to them. As to why the triangle moved and not the rectangle when you used the Smart Dimension function, it's hard to say. It may be because of which order the entities were selected, but there could be other reasons.
Many thanks Glenn, for your time. I should say that I worked through the tutorials and now I'm out on my own. The most relevant tutorial to what I am trying to do is "Blocks, advanced: crane", where a skeleton sketch is built up of a mechanism, to check motion between the parts. The main point of the tutorial is that parts have to be created as blocks in order to move in relation to one another.
In my attempt at flying solo, things seem to go ok at first, when I draw and dimension the triangle and rectangle prior to any creation of blocks. So before "blocking", the lower left apex of the triangle seems happy to stick to the origin, from the moment I began the sketch by starting the triangle's horizontal base line at a snap-to at the origin. Indeed before creating the block, the drawing is well-behaved and I can get it fully defined and use smart dimensions without problems. If I edit the distance between rectangle and triangle, the triangle stays put, the rectangle moves instead, as I expect. However, as soon as the triangle and rectangle are created together as a block (the "engine base") the sketch instantly becomes undefined. And, if I want to edit the block to change the distance between rectangle and triangle, the triangle no longer stays put, as per my original post. So, when the sketch gets more complex with more blocks created for various lever arms and so on, changing the base, and having the whole foundation move around under the mechanism creates havoc.
I guess the problem is that once the 'engine base' block is created, the initial connection it had to the origin is lost. If that is right, then I have to find out how to anchor a certain point of the block in relation to the origin, such that any further modifications to the block cannot shift that point. My use (or over-use) of the fix relation seen in my screenshot was an attempt to hold down the triangle after I had created it as a block.
I can see that in this case, to avoid this problem, perhaps I should just make the rectangle as another block, or else work out all my dimensions before I sketch so that there are no edits. But I had hoped to learn from this example some general methods of "Layout sketching" so that in future I can implement top-down methods in building assemblies. I want to learn fundamental disciplines without bad habits.
Thanks for the added explanation. I rarely use Layout sketches and never use sketch blocks, so I'm afraid I can't be of much help. Maybe someone else will see this who can.
At first glance this model has well defined datum and reference positions. The rectangle is still underdefined. It's hard to tell what is going on with your model as I can't see how you defined your other relationships. It's especially unclear around the apex of the triangular block. How does the rectangle relate to the linkage in the background? Could you post the model?