If f:[3, ∞) → A defined by f(x)=x^2 is an onto function, then A is :

f : [3, ∞) → A

f is onto so every image in A must have atleast one pre-image

$f(x) = x^2$

for boundary conditions

$f(3) = 9$           $\lim\limits_{x \rightarrow \infty} f(x) \rightarrow \infty$

so   A = [9, ∞)