I need to minimize

$\displaystyle \frac{1}{||x||^2}||ax-\mu x||^2$,

where x, a and $\displaystyle \mu$ are vectors in $\displaystyle R^n$.

My Idea was to take the derivative, set it to zero and solve for x, but I'm not good in matrix calculus. I would be extremely thankful if somebody could help me here!