Answer
$\color{blue}{(y^2+9)(y+3)(y-3)}$
Work Step by Step
Note that $y^4=(y^2)^2$ and $81=9^2$.
Thus, the given binomial is equivalent to:
$=(y^2)^2-9^2$
Factor using the formula $a^2-b^2=(a+b)(a-b)$ where $a=y^2$ and $b=9$ to obtain:
$=(y^2+9)(y^2-9)
\\=(y^2+9)(y^2-3^2)$
Factor the second binomial factor using the same formula above with $a=y$ and $b=3$ to obtain:
$=\color{blue}{(y^2+9)(y+3)(y-3)}$
