Answer
$36a^2-12ab+b^2$
Work Step by Step
Using $(a+b)^2=a^2+2ab+b^2$, or the square of a binomial, the given expression, $
(6a-b)^2
,$ is equivalent to
\begin{array}{l}\require{cancel}
(6a)^2+2(6a)(-b)+(-b)^2
\\\\=
36a^2-12ab+b^2
.\end{array}