The answer is B. Multiply
Matter is anything that takes up space and has mass.
48 inches is 4 feet.
36 inches is 3 feet.
The question is asking for the area in square feet of a rectangle that is 4 feet long by 3 feet wide. That area is the product of these dimensions:
(4 ft)×(3 ft) = 12 ft²
D=426-48(21)
I got 18 days.
So, D=18
Hope this helps! :)