# How do you multiply (9-w)(9+w)?

$81 - {w}^{2}$
This is something you must keep in mind: for every $a$ we have
$\left(a - w\right) \left(a + w\right) = {a}^{2} - a w + a w - {w}^{2} = {a}^{2} - {w}^{2}$
In this case $a = 9$ so $\left(9 - w\right) \left(9 + w\right) = {9}^{2} - {w}^{2} = 81 - {w}^{2}$