Q. The number $\displaystyle 6x^2 + 6x$(x is a natural number) is always divisible by

a. 6

b. 18

c 12

d 6 and 12 both

I tried to solve it using a rule :- $\displaystyle (a^n + b^n)$ is always divisible by (a+b), (a-b) if n is even. In this case n is even and odd respectively. What approach should i adopt ?