Answer
The required equation is $x=kz\left( y+w \right)$ and the value of $y$ is $\frac{x-kzw}{kz}$.
Work Step by Step
As the value of $x$ varies directly as $z$ and $\left( y+w \right)$ , we have
$x=kz\left( y+w \right)$
Where $k$ is a constant
Now, solve the above equation for $y$.
$\begin{align}
& x=kzy+kzw \\
& kzy=x-kzw \\
& y=\frac{x-kzw}{kz}
\end{align}$
