[Calculus 1] Finding absolute extreme values on closed interval

hey there, I'm having trouble finding the absolute extrema with the given problem f(x)=-3/x^2; 0.5<=x<=5. I'm stuck on the first step finding the critical points, I took the derivative of f(x) by using the quotient rule and I got f'(x) = 6x/x^4 after I found the derivative I set it equal to 0 and the answer I'm getting is x=0, I don't know how to continue finding the absolute extreme with x =0 (it's not inside the closed interval), I know the second step after finding the critical point is to plug it into f(x) but that would just give me 0. The answer according to the key should be abs max @ (5,-3/25) and abs min @ (0.5, -12). Thanks!

Edit: After rethinking about it can I consider 5 and 0.5 as critical points?