A system is made up of point masses P1 at position (-1 m, 5 m, 7 m), P2 at position (3 m, 3 m, 3 m) and P3 at position (9 m, -5 m, -2 m). Calculate the center of mass of the system if:

(a) each of the point masses have the same mass, and

(b) m₁ = 2m₂ = 4m₃