
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

A = 1/2 bh = 1/2 (3)(4) = 12/2 = 6
answer is B. 6 square units
So, this is a rate problem, so the forumla we would get is
2.5g=30
(Note this is not one of the choices), so if we look, there is an equivelent equation.
If we divide both sides by g we get
2.5=30/g
B is the correct answer