find the volume of solid bound by planes x=0 y=0 z=0 and x+y+z=7

under tripod

area of base = 49/2
find area of section at constant z
x = y = 7-z
area = xy/2 = (7-z)^2/2
integrate area dz from 0 to 7
(1/2) integral[ 49 -14z + z^2]dz from 0 to 7