The inverse of a diagonal matrix is obtained by replacing each element in the diagonal with its reciprocal.
Since,
$\mathit{X}=\left[\begin{array}{ccc}2& 0& 0\\ 0& 3& 0\\ 0& 0& 4\end{array}\right]$
Therefore
${\mathit{X}}^{-1}=\left[\begin{array}{ccc}\frac{1}{2}& 0& 0\\ 0& \frac{1}{3}& 0\\ 0& 0& \frac{1}{4}\end{array}\right]$