Consider the surface xyz = 20.

Question

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.

This is the answer i got for part b: 20(x-1) + 5(y-4) + 4(z-5) = 0. Which is correct. But i'm not sure how to get part a.