I'm supposed to find the second derivative of $\displaystyle (x^2+5)^8$

What I got was: $\displaystyle f(u)=u^8, u(x)=(x^2+5)$

$\displaystyle (8(x^2+5)^7)(2)=16(x^2+5)^7$

$\displaystyle f(u)=16(u)^7, u(x)=(x^2+5)$

$\displaystyle (16)7(x^2+5)^6(2)=224(x^2+5)^6$

but the back of the book says it's: $\displaystyle 80(x^2+5)^6(3x^2+1)$

Any help would be much appreciated!