TeX source:
\begin{array}{rcl} f'_{yy}(x,y,z) &=& 150z\cdot \left(-\dfrac{1}{2}\right)(x+yz)^{-\frac{1}{2}-1}\cdot z \\\\ &=& -75z^2{(x+yz)^{-\frac{3}{2}}}\\\\&=& -\dfrac{75z^2}{\left(\sqrt{x+yz}\right)^3}\end{array}