Factor polynomials: quadratic methods (challenge)

The rectangle below has an area of $3k^2+12k-7kn-28n$3, k, squared, plus, 12, k, minus, 7, k, n, minus, 28, n square meters and a length of $3k-7n$3, k, minus, 7, n meters.

$\text{Width} =$start text, W, i, d, t, h, end text, equals meters