Chapter 13, Problem 19RE

Suppose that z is a difierentiable function of x and y with

$\frac{\partial z}{\partial x}(1,2)=4\text{and}\frac{\partial z}{\partial y}(1,2)=2$

If $x=x(t)\text{and}y=y(t)$ are differentiable function of t with $x(0)=1,y(0)=2,x\text{'}(0)=-\frac{1}{2},\text{and}\text{(under composition ) }z\text{'}(0)=2,\text{find}\text{y'(0)}\text{.}$