TeX source:
\begin{array}{rcl} f'_{xy}(x,y,z) &=& \dfrac{-16yz^2 \cdot\left(17x^2+y^2\right)^2-\left(136x^2z^2-8y^2z^2\right)\cdot 2(17x^2+y^2)\cdot 2y}{\left(\left(17x^2+y^2\right)^2\right)^2} \\\\&=& \dfrac{\left(17x^2+y^2\right)\left[-16yz^2 \cdot\left(17x^2+y^2\right)-\left(136x^2z^2-8y^2z^2\right)\cdot 2\cdot 2y\right]}{\left(17x^2+y^2\right)^4} \\\\&=& \dfrac{-16yz^2 \cdot\left(17x^2+y^2\right)-\left(136x^2z^2-8y^2z^2\right)\cdot 2\cdot 2y}{\left(17x^2+y^2\right)^3} \\\\&=& \dfrac{-272x^2yz^2-16y^3z^2-544x^2yz^2+32y^3z^2}{\left(17x^2+y^2\right)^3} \\\\&=& \dfrac{-816x^2yz^2+16y^3z^2}{(17x^2+y^2)^3}\end{array}