how to setup this problem?

Question

If x=y≠4, and x=20-y, then x^2 - y^2 = ?

I got the answer as 0 first, because I read the first line as x=y and y is not equal to 4....but if x = y, then the answer is 0 and that is not one of the choices.

I finally got the answer as 80...which is correct...but dont get the logic?

Answer 1

x=y. neither one is equal to 4.
x=20-y
x=12 therefore y=8
x^2=144
y^2=64
144-64=80

Answer 2

It looks to me like the 1st line should read:
x - y = 4
then
x + y = 20
2x = 24
x = 12
t = 8
12^2 - 8^2 = 144 - 64 = 80