Question

Can anybody help me with this?
Consider the surface xyz = 20.
A. Find the unit normal vector to the surface at the point (1, 4, 5) with positive first coordinate.
(___,___,___)

B. Find the equation of the tangent plane to the surface at the given point. Express your answer in the form ax + by + cz + d = 0, normalized so that a = 20.
= 0.

For part b i got 20(x-1) + 5(y-4) + 4(z-5) which is correct. And i got (x-1),(y-4)/4,(z-5)/5 for a normal line. But i'm not sure how to get part A...