How do I solve this? (functions, image sets)

I'm having some trouble with the following question. Help is greatly appreciated.

Let f : Z → Z be the function defined by f(x) = x^2.
(a) Let A = {3,4,7,9}. Find f(A).
(b) Let B = {25,30,36,47,100,144}. Find f^−1 (B).

a) Let A = {3,4,7,9}. Find f(A).

f(A) = {9,16,49,81}

(b) Let B = {25,30,36,47,100,144}. Find f^−1 (B).

y=f(x)=x^2
y=x^2 resolve for x
x=+/-sqrt y
f^−1 (B)= {+/-5,+/-sqrt30, +/-6, +/-sqrt47, +/-10, +/-12}