290. Excluding the unknown $y$ from the system of equations

$$
\begin{aligned}
& a_{1} x+b_{1} y=c_{1} \\
& a_{2} x+b_{2} y=c_{2}
\end{aligned}
$$

we obtain

$$
x=\frac{c_{1} b_{2}-c_{2} b_{1}}{a_{1} b_{2}-a_{2} b_{1}}
$$

Write, without new calculations, the formula for $y$.