You get everything you need from factoring the last expression:

The left side is a difference of squares, and we get another difference of squares upon factoring. We end up with

Plug in everything you know and solve for
:


has gradient

which at the point (-1, 4, 3) has a value of

I'm not sure what the given direction vector is supposed to be, but my best guess is that it's intended to say
, in which case we have

Then the derivative of
at (-1, 4, 3) in the direction of
is

Number 3 is 9 not negative 10
The third one because 2(15+5)=2(20)=40