Paragraph for Questions 11 and 12
Let a,r,s,t be nonzero real numbers. Let P(at2,2at), Q, R(ar2,2ar) and S(as2,2as) be distinct points on the parabola y2=4ax. Suppose that PQ is a focal chord and lines QR and PK are parallel, where K is the point (2a,0).
The value of r is