1. The problem statement, all variables and given/known data Schaum's vector analysis prob 4.65 Find the values of constants a,b,c so that directional derivative of phi=axy^2+byz+cz^2x^3 at (1,2,-1) has a maximum of magnitude 64 in a direction parallel to the z axis. 2. Relevant equations 3. The attempt at a solution Directional derivative is maximum along grad(phi). After taking grad(phi) at (1,2,-1) I got (4a+3c)i+(4a-b)j+(2b-2c)k I don't know how to make use of the 'direction parallel to z axis' condition.