Let $x = \dfrac{ky}{z^2}$, where $k$ is a constant.
Let $x_0$, $y_0$ and $z_0$ be the original value of $x$, $y$ and $z$ respectively.
Let also $x_1$, $y_1$ and $z_1$ be the new values of $x$, $y$ and $z$ respectively.
$\begin{array}{rcl}
y_1 & = & y_0 \times (1-10\%) \\
& = & 0.9y_0
\end{array}$
$\begin{array}{rcl}
z_1 & = & z_0 \times (1+20\%) \\
& = & 1.2z_0
\end{array}$
Hence the percentage change of $x$
$\begin{array}{cl}
= & \dfrac{x_1 – x_0}{x_0} \times 100\% \\
= & \dfrac{\frac{ky_1}{z_1^2} – \frac{ky_0}{z_0^2}}{\frac{ky_0}{z_0^2}} \times 100\% \\
= & \dfrac{\frac{k(0.9)y_0}{(1.2z_0)^2} – \frac{ky_0}{z_0^2}}{\frac{ky_0}{z_0^2}} \times 100\% \\
= & \dfrac{\frac{ky_0}{z_0^2}( \frac{0.9}{(1.2)^2} – 1 )}{\frac{ky_0}{z_0^2}} \times 100\% \\
= & (0.625 – 1) \times 100\% \\
= & -37.5\%
\end{array}$
Therefore, $x$ is decreased by $37.5\%$.
