y-9/y=10 what's why

Multiply through by y:

y² - 9 = 10y

Subtract 10y from both sides:

y² - 10y -9 = 0

This is now in standard quadratic equation form.

$${{\mathtt{y}}}^{{\mathtt{2}}}{\mathtt{\,-\,}}{\mathtt{10}}{\mathtt{\,\times\,}}{\mathtt{y}}{\mathtt{\,-\,}}{\mathtt{9}} = {\mathtt{0}} \Rightarrow \left\{ \begin{array}{l}{\mathtt{y}} = {\mathtt{5}}{\mathtt{\,-\,}}{\sqrt{{\mathtt{34}}}}\\
{\mathtt{y}} = {\sqrt{{\mathtt{34}}}}{\mathtt{\,\small\textbf+\,}}{\mathtt{5}}\\
\end{array} \right\} \Rightarrow \left\{ \begin{array}{l}{\mathtt{y}} = -{\mathtt{0.830\: \!951\: \!894\: \!845\: \!300\: \!5}}\\
{\mathtt{y}} = {\mathtt{10.830\: \!951\: \!894\: \!845\: \!300\: \!5}}\\
\end{array} \right\}$$