Question

18=20+x I need to know what x= but wouldn't it be subtraction

Answer 1

18=20+x
-20 -20
x= 18-20
x= -2

Answer 2

Please !  Don't make something hard out of something easy. 
It makes math look bad.

Simply take the original equation               18 = 20 + x

You want 'x' all alone on one side of the
equation, so subtract 20 from each side:    -2 = x