Answer
$7-2\sqrt{10}$
Work Step by Step
Using $(a-b)^2=a^2-2ab+b^2$ or the square of a binomial, then,
\begin{array}{l}
\left( \sqrt{5}-\sqrt{2} \right)^2
\\=
(\sqrt{5})^2+2(\sqrt{5})(-\sqrt{2})+(-\sqrt{2})^2
\\=
5-2\sqrt{10}+2
\\=
7-2\sqrt{10}
.\end{array}