# What is the standard form of  (x+3y)(3x-y) ?

$3 {x}^{2} + 8 x y - 3 {y}^{2}$
$\left(x + 3 y\right) \left(3 x - y\right) = 3 {x}^{2} + 9 x y - x y - 3 {y}^{2}$
$= 3 {x}^{2} + 8 x y - 3 {y}^{2}$