An electric field line is generated at charge $q_1$  and terminates at $q_2$  as shown. The equation of the line can be

$q_1(1-\frac{x}{\sqrt{x^2+y^2}})-q_2(1-\frac{\mathcal{l}-x}{\sqrt{(\mathcal{l}-x^2)+y^2}})=q_1(1-\cos \alpha )$

$q_1(1+\frac{x}{\sqrt{x^2+y^2}})-q_2(1-\frac{\mathcal{l}-x}{\sqrt{(\mathcal{l}-x^2)+y^2}})=q_1(1-\cos \alpha )$

$q_1(1+\frac{x}{\sqrt{x^2+y^2}})+q_2(1-\frac{\mathcal{l}-x}{\sqrt{(\mathcal{l}-x^2)+y^2}})=q_1(1-\cos \alpha )$

$q_1(1-\frac{x}{\sqrt{x^2+y^2}})+q_2(1-\frac{\mathcal{l}-x}{\sqrt{(\mathcal{l}-x^2)+y^2}})=q_1(1-\cos \alpha )$