Answer
$(a-6)(a^2+6a+36)$
Work Step by Step
RECALL:
$a^3-b^3 = (a-b)(a^2+ab+b^2)$
Write 216 as a cube to have:
$=a^3-6^3$
Factor the difference of two cubes using the formula above with $a=a$ and $b=6$ to have:
$=(a-6)(a^2+a(6) + 6^2)
\\=(a-6)(a^2+6a+36)$
