\begin{subequations}
  \begin{align}
    a_1 & = \frac{(y - c_2) c_1 \pm \sqrt{(1+c_1^2) x^2 - (y - c_2)^2}}{1+c_1^2} \; , \\
    b_1 & = \frac{ y- c_2 \mp c_1 \sqrt{(1+c_1^2) x^2 - (y - c_2)^2}}{1+c_1^2} \; ,
  \end{align}
\end{subequations}
