Let f(x, y) = x^2y^3 + xy.
Is there a direction at (-1; 2) in which the rate of change of f is equal to 18?
Justify your answer.
The Attempt at a Solution
plugging this into the directional derivative formula, i get
18 = -v1 + 13v2, where v1 and v2 are the components of the vector in whose direction the rate of change is 18.
But i do not know where to go from here...