Answer
$a^{2}-36b^{2}$
Work Step by Step
$(a-6b)(a+6b)$
...identify which terms correspond to $a $and $b $
in the rule for the product of a sum and difference ($(a+b)(a-b)=a^{2}-b^{2}$).
$a=a,\ b=6b$
...substitute for $a $and $b $ in the rule
$(a-6b)(a+6b)=a^{2}-(6b)^{2}$
...simplify.
=$a^{2}-36b^{2}$
