Answer
$\dfrac{a^{2}}{4}-16y^{2}$
Work Step by Step
$\Big(\dfrac{a}{2}+4y\Big)\Big(\dfrac{a}{2}-4y\Big)$
To evaluate this product, use the following formula:
$(m+n)(m-n)=m^{2}-n^{2}$
Here, $m=\dfrac{a}{2}$ and $n=4y$
$\Big(\dfrac{a}{2}+4y\Big)\Big(\dfrac{a}{2}-4y\Big)=\Big(\dfrac{a}{2}\Big)^{2}-(4y)^{2}=\dfrac{a^{2}}{4}-16y^{2}$
