If anyone could help me with this problem, it would be most appreciated.

Find an equation for the plane tangent to the sphere x^2+y^2+z^2=81 at (1,4,8).

I have tried finding the vector perpendicular to [1,4,8] but I end up with a+4b+8c=0 and can't figure out a way to find a, b, and c.

Thanks.