Urgent, please help me

Question

Find the volume of a pyramid with a height of 20 feet whose base is an equilateral triangle with sides measuring 10 feet in length.

I should evaluate the volume using an integral. The answer is v=sqrt3/16 integral from 0 to 20 (20-x)^2 dx, but I can't understand why the answer looks like this. Please help me

Answer 1

v = ∫₀²⁰100√3/4(20 - x)²/400dx = - √3(20 - x)³/48₀²⁰ = 500√3/3

X is distance from base of height to the cross section