TeX source:
\begin{array}{rcl}f'_z(x,y,z) &=& 300\cdot\dfrac{1}{2}\left(x+yz\right)^{\frac{1}{2}-1}\cdot y \cr \cr&=& 150y\left(x+yz\right)^{-\frac{1}{2}} \cr \cr&=& \dfrac{150y}{\sqrt{x+yz}} \end{array}