Area of the base is 100 square feet and the height is 30 find the volume

The equation for volume is
v = length * width * height

the equation for the area of a square is
a = length * width

since length * width is in the top equation we can replace it with area like this
v = (length * width) * height
v = (area) * height
v = 100 * 30
v = 3000 ft^3