Let b be the side of the pyramid base (6 metres) and h be the height of the pyramid (10 metres).
Any horizontal slice through the pyramid will produce a square with side
b(h-z)/h where z is the height above the bottom.
The density is proportional to the distance x from the apex so:-
density = a times x but note that x = h-z
so the mass of a square at a height z above the bottom is its area (side times side) times the density at that level times dz
dm = [b(h-z)/h]**2 times a(h-z) times dz
Mass is integral from z=0 to z=h of
(a*b**2/h**2) (h-z)**3 dz
Which I assume you can do