What is the missing binomial?

a) [[$ x^2 + 8x + 15 = (x + 3) $]]()

b) [[$ x^2 - 10x + 21 = (x - 3) $]]()

c) [[$ x^2 + x - 12 = (x - 3) $]]()

d) [[$ x^2 - x - 6 = (x - 3) $]]()