A=7d+13
A=7(4)+13
A= 28+13
A=41

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

First let's get rid of the parenthesis:
12 - 4x + 3x = 4 + 10 + 2x
Now group the x's on the left and the numbers on the right
-4x + 3x - 2x = 4 + 10 - 12
...and simplify
-3x = 2
divide by -3 to isolate x
x = -2/3
Answer:
x = 2/5
Step-by-step explanation:
5x=2
Divide each side by 5
5x/5 = 2/5
x = 2/5