Please find the inverse of f(x) = x^2 , x is less than and equal to 0.

I got f^-1(x) = sqrt(x) . But what do I do about the exception "x is less/equal to 0"

Also, the question also states verify that (f o f^-1)(x) = (f^-1 o f)(x) = x . What does this mean? What does the "o" mean? Does it mean to multiply? I've never seen that notation before.

Thank you very much. If anything is unclear please note, and I'll try my best to clarify. Thank you.