If $z = f ( x , y )$ And $x = s + t , y = s - t$

Essay

If $z = f ( x , y )$ and $x = s + t , y = s - t$ , then simplify $\left(\frac{\partial z}{\partial s}\right)\left(\frac{\partial z}{\partial t}\right)$ in terms of $f _ { x }$ and $f _ { y }$ .