Tricky ellipse question

[Ax3]

Hey guys anyone know how to go about part b of this question?

http://img171.imageshack.us/img171/200/questionmx.png

[xZero]

so from part a) we know the derivative of the ellipse, b) is saying that at the point of intersection (x,mx) the gradient of the ellipse is perpendicular to y=mx(in other words the line y=mx is the normal to the ellipse), which means that -1/m = the derivative of the ellipse at the point (x,mx), now set that equation up and solve for m.