The equation of the tangent to

at the point $P$ is

Find the coordinates of $P$ and the value of $c.$

Whilst you can do this with the discriminant, differentiation is probably easier.

Notice that the gradient of the tangent is $6,$ and so, at $P,$

You also know that $P$ lies on both the line and the parabola, so the coordinates of $P$ satisfy both of their equations.