Solve Y=x^2-18 for x

y=X^2-18

since 18 doesn't contain the variable add 18 to both sides to get

18+y = x^2

then take square root to get just x

so square root (18+y) = x

for complete solution use both negative and positive answers so

x = - square root (18+y) & x = square root (18+y)