Answer:
x(3+28y)
Step-by-step explanation:
21x-18x+28xy
3x+28xy
x(3+28y)
Make a change of coordinates:


The Jacobian for this transformation is

and has a determinant of

Note that we need to use the Jacobian in the other direction; that is, we've computed

but we need the Jacobian determinant for the reverse transformation (from

to

. To do this, notice that

we need to take the reciprocal of the Jacobian above.
The integral then changes to

I’m guessing C, but I’m not for sure
day 5 has the highest visits and day 3 has the lowest visits