HELLO :
$\displaystyle \begin{array}{l}
\frac{{ - y}}{{ - a}} = \frac{a}{x} \\
\left( { - y} \right) \times \left( x \right) = \left( { - a} \right) \times \left( a \right) \\
\left( { - y} \right) \times \left( x \right) = - a^2 \\
y \times x = a^2 \\
\end{array}$
divide by x :
$\displaystyle \frac{{y \times x}}{x} = \frac{{a^2 }}{x}
$
conclusion $\displaystyle y = \frac{{a^2 }}{x}
$