Hello, I am having trouble with question 4 on this past paper and wondered if anyone could help me with it? It's to do with directional derivatives....

Hello, I am having trouble with question 4 on this past paper and wondered if anyone could help me with it? It's to do with directional derivatives....

Thank you

The direction in which $\displaystyle \Psi$ changes at the greatest rate is given by $\displaystyle \nabla \Psi$. Calculate it. Now calculate the directional derivative of $\displaystyle \phi$ in that direction.