Please teach how to solve Only part b

Especially Please teach should part b answer write as unit vector, or the f(3,4,5) itself ?

6. The electrical potential (voltage) in a certain region of space is given by the function V(x, y,z) = 5x2 3xy+ xyz. a. Find the rate of change of the voltage at point (3,4,5) in the direction of the vector (LL1). b. In which direction does the voltage change most rapidly at point (3,4,5)? 0. What is the maximum rate of change of the voltage at point (3,4, 5)