Ok, the question is for what value of $\displaystyle a$ is the expected value $\displaystyle E([aX- \frac{1}{a}]^2)$ minimum, given that $\displaystyle \sigma ^2 >0 $ and $\displaystyle a$ $\displaystyle > 0$

I don't want the answer, I just want a push in the right direction. How do I find the minimum of the expected value?

Thanks!