Question

Find the value of x for which ABCD must be a parallelogram

Answer 1

x+3=2x-1
x=2x-4
-1x=-4
x=4

Answer 2

Parallelograms have opposite sides that are equal, so you can set BC and AD equal to each other to find the value of x if the figure is a parallelogram. 
x + 3 = 2x - 1
3 = x - 1
4 = x

Now check
4 + 3 = 2(4) - 1
7 = 7

The value of x that makes ABCD a parallelogram is x = 4.

Hope this helps =)