The answer is 1 bag.
length of the box is = 7 feet
width is = 9 inches
and height is = 6 inches
Each bag of manure he bought contains = 2.625 cubic feet of manure
The volume (V) of the rectangular box is:
V = l × w × h
where l is length, w is width and h is height
Now you see that the unit of volume is cubic feet, first convert width and height into feet which are in inches
width = 9 inches = 0.75 feet
height = 6 inches = 0.5 feet
Now, V = l x w x h
=7 x 0.75 x 0.5
= 2.625
Now Each bag of manure he bought contains 2.625 cubic feet of manure which is equal to the volume of box.
Thus, the answer is only 1 bag will fill the box completely.