Finding directrix of a parabola Let $3x – y – 8 = 0$ be the equation of tangent to a parabola at the point $(7, 13)$. If the focus of the parabola is at $(– 1, – 1)$, then the equation of its directrix is?

HINT:
Let the equation of the directrix be $y=mx+c$
So, the equation of the parabola :  $$(x+1)^2+(y+1)^2=\dfrac{(mx-y+c)^2}{m^2+1}$$
First of all, it passes through $(7,13)$
Find the equation of the tangent at $(7,13)$ and compare with $3x–y–8=0$
