Split the second term in 3x^2 + 31x + 36 into two terms
3x^2 + 27x + 4x + 36
Factor out common terms in the first two terms, then in the last two terms
3x(x + 9) + 4(x + 9)
Factor out the common term; x + 9
(x + 9)(3x + 4)
