P is a point t on the parabola $y^2=4ax$ and PQ is a focal chord. PT is a tangent at P and QN is a

Question

Question :

P is a point t on the parabola $y^2=4ax$ and PQ is a focal chord. PT is a tangent at P and QN is a normal at Q. Find the minimum distance between PT and QN.

Solution :

Since the minimum distance between the tangent PT and QN is the common normal between the two.

But I am not getting any further clue on this please suggest thanks...