This is what the parallelogram will look like, and since BC and AD should equal the same length, you set them equal to each other to find x.
3x+15 = 5x+3
12 = 2x
6 = x
Now that you have found x you plug it in to the equation of length AD.
5(6)+3 = 30+3 = 33
Your answer should be 33.
