$y=\dfrac{x^2+1}{3x-7} \Rightarrow x^2+1=3yx-7y \Rightarrow x^2-3yx=-7y-1 \Rightarrow $
$\bigg(x-\dfrac{3y}{2} \bigg )^2=\bigg (\dfrac{3y}{2} \bigg)^2-7y-1 \Rightarrow x-\dfrac{3y}{2}=\mp \sqrt{\bigg ( \dfrac{3y}{2} \bigg )^2-7y-1} \quad \Rightarrow $
$x=\dfrac{3y}{2} \mp \sqrt{\bigg ( \dfrac{3y}{2} \bigg )^2-7y-1}$