find the general gradient of the point of intersection which must be:
2x/a^2+dy/dx(2y/b^2)=o
dy/dx=-x(b^2)/y(a^2)
if (x1,y1) is the xternal pt
(y-y1)=-x(b^2)/y(a^2)[x-x1]
x^2/a^2+y^2/b^2=x.x1/a^2+y.y1/b^2 [sub in the x1,y1]
Although this will onli give u one eqn.