volume of tetrahedron = 1/3 (base area) * height

if the volume of the parallelepiped corresponding to the volume of the tetrahedron is pv, then the volume of the tetrahedron (tv) = pv/6

(x1,y1,z1) is the vertex p

(x2,y2,z2) is the vertex q

(x3,y3,z3) is the vertex r

(x4,y4,z4) is the vertex s.