Answer
$$(y-7z)(y+3z)$$
Work Step by Step
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,
$$y^2-4yz-21z^2=(y-7z)(y+3z)$$
