Answer
See the explanation below.
Work Step by Step
The chain rule states that when $z=f(x,y)$ is a differentiable function of $x$ and $y$ (where $x=g(s),y=h(s)$ are function of a single variable let us say $s$), then:
$\dfrac{dz}{dt}=\dfrac{\partial f}{\partial x}\dfrac{dx}{dt}+\dfrac{\partial f}{\partial y}\dfrac{dy}{dt}$
Also, the chain rule states that when a function $z=f(x,y)$ is a differentiable function of $x$ and $y$ where $x=g(s),y=h(s)$ are functions of two variables (let us say $s,t$), then:
$\dfrac{\partial z}{\partial s}=\dfrac{\partial z}{\partial x}\dfrac{\partial x}{\partial s}+\dfrac{\partial z}{\partial y}\dfrac{\partial y}{\partial s}$
and
$\dfrac{\partial z}{\partial t}=\dfrac{\partial z}{\partial x}\dfrac{\partial x}{\partial t}+\dfrac{\partial z}{\partial y}\dfrac{\partial y}{\partial t}$
