ok so choose a point on the parabola x^2=4ay: P(2ap,ap^2).
Now let's find the normal at that point.
y=x^2/4a
dy/dx=2x/4a
x=2ap, dy/dx=p so gradient of normal is -1/p
y-ap^2=-1/p (x-2ap)
Sub (0, Ka) into the equation[since you are given that the normals pass through (0,Ka)]
Ka-ap^2=...