The answer is 1 bag.
First, watch out, the units are different! Next, let's first calculate a volume of a box.
The volume (V) of the rectangular box is:
V = l × w × h
(l - length, w - width, h - height)
It is given:
l = 7 feet = 84 inches
w = 9 inches
h = 6 inches
Therefore,
V = 84 · 9 · 6 = 4.536 in³
We know that each bag of manure contains 2.625 cubic feet of manure.
2.625 ft³ = 4536 in³
Now, make a proportion:
If 1 bag contains 4536 cubic inches of manure, how many bags will fill the box of 4536 cubic inches:
1 : 4536 = x : 4536
⇒ x = 4536 * 1 / 4536 = 1
Therefore, only 1 bag of manure will fill the box completely.