You simply do it the same way as a positive number, just with a negative sign in front :)
Answer:
A) 48 in : initial height of the water
B) 40.5 in : height of the water after 5 days
C) 32 days
D) Yes, should be restricted.
Domain: 0 ≤ d ≤ 32
Step-by-step explanation:

A)

48 in is the initial height of the water
B)

40.5 in is the height of the water after 5 days
C) when the pool is empty, h = 0.
So set the equation to zero and solve for d.

D) Yes, the domain should be restricted because when d > 32, h < 0 and the height of the water in the pool cannot be negative
Domain: 0 ≤ d ≤ 32
4x345=1380
3000-1380=1621
Answer= Denise would have 1,621 water bottles left