Split the second term in 9s^2 - 36s + 35 into two terms
9s^2 - 15s - 21s + 35
Factor out common terms in the first two terms, then in the last two terms
3s(3s - 5) - 7(3s - 5)
Factor out the common term 3s - 5
(3s - 5)(3s - 7)
