#### Answer

$-5$

#### Work Step by Step

Using $(a+b)(a-b)=a^2-b^2$, or the product of the sum and difference of like terms, and the properties of radicals, the given expression, $
(\sqrt{10}-\sqrt{15})(\sqrt{10}+\sqrt{15})
,$ is equivalent to
\begin{array}{l}\require{cancel}
(\sqrt{10})^2-(\sqrt{15})^2
\\\\=
10-15
\\\\=
-5
.\end{array}