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 changes at the greatest rate is given by . Calculate it. Now calculate the directional derivative of in that direction.