welcome to the forum
What exactly you mean by "unite"
Can you post a picture or screen shot of what you are looking for?
Tangent, or end point to end point, or both?
Sounds like you need to work through the tutorials in the Help section.
If you mean "...how can unite a line and an arc into one sketch element?...", use 'Fit Spline'. This will overlay a spline onto your line and arc. If it's only one line and one arc, you'll need to uncheck the 'Closed spline' option.
I'm Sorry guys
Actually I committed a mistake while creating a sketch
But now it's ok
Thanks to all of you for trying to solve my problem
I was actually asking for a command similar to "REGION" Which is used in AutoCAD