What have you done so far? Why not start by eliminating the variable x? Do you know how to do that? Hint: use multiplication / division of rows and then addition / subtraction.
Answer:
a)
.
b) 
Step-by-step explanation:
Given a function
, this function has the following gradient:
.
(a) find the gradient of f
We have that
. So


.
.

(b) find the directional derivative of f at (2, 4, 0) in the direction of v = i + 3j − k.
The directional derivate is the scalar product between the gradient at (2,4,0) and the unit vector of v.
We have that:

.

The vector is 
To use v as an unitary vector, we divide each component of v by the norm of v.

So

Now, we can calculate the scalar product that is the directional derivative.

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Answer:
C
Step-by-step explanation:
Under a reflection in the y- axis
a point (x, y ) → (- x, y ), thus
A(3, 4 ) → A'(- 3, 4 )
Since the point A and it's reflection lie on a horizontal line then
distance between = | 3 + 3 | = | 6 | = 6
The first one is Circle and the second one is triangle