How to constrain two angles to be equal? (drawing a trapezoid)

Link values or Global Variables can do this.  Take a look a the following link from the 2014 online help, it will walk you though the process.

http://help.solidworks.com/2014/English/SolidWorks/sldworks/t_linking_dimension_values.htm

Here is the alternative method using Global Variables.

http://help.solidworks.com/2014/English/SolidWorks/sldworks/t_link_dimensions_use_global_variables.htm

use symmetry

Nathan,

Not sure if this is the best way, but I usually use a centerline and symmetry relationship to achieve this.

Draw the diagonals and constrain them to be equal.

I like the global variable method. Another method is to use Mirror to make the other half of the trapazoid.