## Calculus with Applications (10th Edition)

$$(y-7z)(y+3z)$$
In order to factor the expression: $$y^2-4yz-21z^2$$ Consider all the pairs of factors of $-21z^2$. Find the pair that sums to $-4z$: $$-7z\times 3z = -21z^2$$ $$-7z+3z=-4z$$ Therefore, $$y^2-4yz-21z^2=(y-7z)(y+3z)$$