The function y = f(x) has an inverse function y = f^−1 (x). If f(a) = −b then which of the following must be true?

1) f^1(-b)=-a
2 f^1(1/a)=-1/b
3) f^1(-b)=a
4) f^1(b)=-a

please explain which answer is correct and why, i really don't understand it!