In order to factorize the given expression, we find to find two numbers p and q such that:
p + q = 5, pq = -36
Clearly,
9 – 4 = 5, 9 (-4) = -36
Therefore,
split 5y as 9y – 4y
Therefore,
y2 + 5y – 36 = y2 + 9y – 4y – 36
= y (y + 9) – 4 (y + 9)
= (y – 4) (y + 9)