If , , and are vectors, how are you defining " " and " "? Is that the dot product? But then it wouldn't make sense to take the norm of which would be a number! Are you sure that and are not scalars?
you can just combine a and $\mu$ into
$c=a-\mu$ and then find $x$ to minimize
$\dfrac {\|c\cdot x\|^2}{\|x\|^2}$
This is clearly minimized if $x$ is orthogonal to $c$
As $c\in \mathbb{R}^n$ there is an (n-1) dimensional subspace containing all these vectors orthogonal to $c$