Do you have a picture would probably make more sense if you added one
33-4(3)
33-12
21
The answer is 21

First, simplify

to

/ Your problem should look like:
Second, multiply both sides by 36. / Your problem should look like:
Third, multiply both sides by -1. / Your problem should look like:
Fourth, switch your sides. / Your problem should look like:

Answer:
C) 21
156-36+36=84/2= 42 <—— the missing length.
Area would be 36x42= 1,512.