The volume of the pyramid is calculated by multiplying the area of the base by the height of the figure. For this item, for the figures to have the same volume,
V = B1H1 = B2H2
Then, we substitute the given values, and since we are not given the shape of the base and the volume of the entire figure, we can just solve it through the way below.
(20 in)(21 in) = (x in)(84 in)
The value of x in the problem is 5 inches.
