Answer
$4a^2+4ab+b^2-12a-6b+9$
Work Step by Step
Using $(a+b)^2=a^2+2ab+b^2$ or the square of a binomial, the given expression, $
[(2a+b)-3]^2
,$ is equivalent to
\begin{array}{l}\require{cancel}
(2a+b)^2+2(2a+b)(-3)+(-3)^2
\\\\=
(2a+b)^2-6(2a+b)+9
\\\\=
(2a+b)^2-12a-6b+9
\\\\=
[(2a)^2+2(2a)(b)+(b)^2]-12a-6b+9
\\\\=
4a^2+4ab+b^2-12a-6b+9
.\end{array}