Find $\displaystyle f''(x)$ if $\displaystyle f(x)=4e^{7x^4}$

Heres my work but I think its wrong b/c I cant finish it

$\displaystyle f'(x)=4e^{7x^4}(28x^3)$

$\displaystyle =112x^3e^{7x^4}$

$\displaystyle f''(x)=112x^3e^{7x^4}(28x^3)+e^{7x^4}(336x^2)$