Answer
$\color{blue}{r(r^2+18r+108)}$
Work Step by Step
With $216=6^3$, the given polynomial is equivalent to:
$=(r+6)^3-6^3$
Factor using the formula $a^3-b^3=(a-b)(a^2+ab+b^2)$ with $a=r+6$ and $b=6$ to obtain:
$=(r+6-6)[(r+6)^2+(r+6)(6) + 6^2]
\\=(r+0)[(r^2+12r+36)+6r+36+36]
\\=r(r^2+12r+36+6r+72)
\\=\color{blue}{r(r^2+18r+108)}$
