Two square pyramids have the same volume. for the first pyramid, the side length of the base is 20 in., and the height is 21 in. the second pyramid has a height of 84 in. what is the side length of the base of the second pyramid?

The volume of the pyramid for a square base can be written as: 

V = 

First, get the volume of the first pyramid
V = ?
s = 20 in.
H = 21 in.

V = 
V = 2800 in³

Since, the volume of two pyramids are the same, we will equate it to equation 2 

V = 2800 in³
s = ?
H = 84 in.

Applying it to the formula:
2800 = 
= s²    ⇒ combine like terms 
100 = s²        ⇒ get the square root of both sides to get the value of s
10 in = s       ⇒side length of the base of the second pyramid