They want you to combine the variables. So
−12x4 + 7x3 + 6x4 − 2x3 =
5x3 − 6x4
So you combine the coefficients, not the variable number, for instance -12+6=-6, and then you simply put the variable in the same place it was. You don't do anything else with it except what I just showed you
hope it helps

Let

The curl is

where
denotes the partial derivative operator with respect to
. Recall that



and that for any two vectors
and
,
, and
.
The cross product reduces to

When you compute the partial derivatives, you'll find that all the components reduce to 0 and

which means
is indeed conservative and we can find
.
Integrate both sides of

with respect to
and

Differentiate both sides with respect to
and




Now

and differentiating with respect to
gives




for some constant
. So

Here we need to calculate common denominator so, LCM of 16 & 24
It would be 48
So, final answer is 48
Hope it helped.
The required relength is half of the length of AC
AC = sqrt(8 - (-2))^2 = 8 + 2 = 10
Therefore, the required length is 5.