This is question 1.6 from Humphrey's "Introduction to Lie Algebras and Representation Theory":
Suppose $x \in \mathfrak{gl}(n,\mathrm{F})$ has $n$ distinct eigenvalues $a_1,\ldots,a_n$. The eigenvalues of the adjoint representation $\mathrm{ad}(x)$ are then precisely the $n^2$ scalars $a_i - a_j$ for $1\leq i,j \leq n$, which of course need not to be distinct.
How do I go on proving this? Any help will be much appreciated