I have a test tomorrow so i started going through all the math problems in my book and i am having problems on 3 in this section

f(x)=x^2/3(x^2-4) on [-1,2]
the problem on this is that i have no idea how to find the derivative of this, I get to f'(x) = x^2/3(2x) + (x^2-4)(2/3x^-1/3), but i dont know how to multiply it out...

then on f(x) = x/x^2+2 on [-1,2]
i get f'(x)= 2/x^1/3-2
this is as far as i get, i don't really know where to go after this..when i set it equal to zero it doesn't pan out right

then the last is f(x)=1/x on (0,infinity)
f'(x)=-x^-2
how would i solve for infinity?