Convert the following equation into intercept form: f(x) = 3x2 + 21x + 36
A f(x)= 3(x – 3)(x – 4)
B f(x) = 3(x – 9)(x – 12)
C f(x) = 3(x + 3)(x + 12)
D f(x) = 3(x+3)(x + 4)