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

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

First step is to get rid of the square root. remember that a square root is the same as raising to the (1/2) power. see below: multiply exponents per exponent rules to get: remember you subtract exponents from the denominator and add exponents in the numerator so: simplify further: so a is the correct choice. if you don't understand this google exponent rules and it will you understand. look at the image results.