$PN$ is an ordinate of the parabola $y^2=4ax$ ($P$ on $y^2=4ax$ and $N$ on  x-axis). A straight line is drawn parallel to the axis to bisect $NP$ and meets the curve in $Q$. $NQ$  meets the tangent at the vertex $A$ in a point $T$ such that $AT=k·NP$, then the value of $k$ is (where $A$ is the vertex)