i need to differenciate :

$\displaystyle

x^3(2x+6)^4

$ using product rule

i got to this stage :

$\displaystyle x^3 \times 8(2x+6)^3 + (2x+6)^4 \times 3x^2 $

but the answer stated in the textbook is $\displaystyle 2x^2(2x+6)^3(7x+9) $

I think its the simplifying which i don't understand provided that the stage i got to is correct.

any help is appreichated.

Thank You