Answer
$4a^2-b^2$
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, $
(2a-b)(2a+b)
,$ simplifies to
\begin{array}{l}\require{cancel}
(2a)^2-(b)^2
\\\\=
4a^2-b^2
.\end{array}