Paragraph: Let a, r, s, t be nonzero real numbers. Let P(a t^ 2 , 2...

Paragraph: Let $a, r, s, t$ be nonzero real numbers. Let $P\left(a t^{2}, 2 a t\right), Q, R\left(a r^{2}, 2 a r\right)$ and $S\left(a s^{2}, 2 a s\right)$ be distinct points on the parabola $y^{2}=4 a x$. Suppose that $P Q$ is the focal chord and lines $Q R$ and $P K$ are parallel, where $K$ is the point $(2 a, 0)$.
Question: If $s t=1$, then the tangent at $P$ and the normal at $S$ to the parabola meet at a point whose ordinate is