Answer
$25x^2-121y^2$
Work Step by Step
Using $(a-b)(a+b)=a^2-b^2$, or the product of the sum and difference of like terms, the given expression, $
(5x-11y)(5x+11y)
,$ is equivalent to
\begin{array}{l}\require{cancel}
(5x)^2-(11y)^2
\\\\=
25x^2-121y^2
.\end{array}