Math Help - Partial derivative

1. Partial derivative

$\frac{\partial f}{\partial y} = - \cos(2 x y z) 2 x z$, then
$\frac{\partial^2 f}{\partial y \partial z} = -2 \frac{\partial}{\partial z} \left( x z\cos(2 x y z) \right)$ then use the product rule and chain rule again.