Find $\frac{dy}{du},\frac{du}{dx},$ and $\frac{dy}{dx}.$

$y=u^{45}$ and $u=5x^4+4x^2$

$\frac{dy}{du}=45u^{44}$

$\frac{du}{dx}=20x^3+8x$

$\frac{dy}{dx}=$