Answer
$4a^2+4ab+b^2-9$
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)-3][(2a+b)+3]
,$ is equivalent to
\begin{array}{l}\require{cancel}
(2a+b)^2-(3)^2
\\\\=
(2a+b)^2-9
.\end{array}
Using $(a+b)^2=a^2+2ab+b^2$ or the square of a binomial, the expression above is equivalent to
\begin{array}{l}\require{cancel}
[(2a)^2+2(2a)(b)+(b)^2]-9
\\\\=
4a^2+4ab+b^2-9
.\end{array}
