how do I find a value of x that will make each proportion true??

Question

12 = x
___ ___
16 20

and
8 = 14
__ ___
12 x

Cross multiply to find x.

Here's the first one; perhaps you can try the second one on your own.

12/16 = x/20

16x = 240

Divide both sides by 16 to get x by itself and solve the problem.

x = 15

Therefore: 12/16 = 15/20

I hope this will help.