CB = B-C = (1,2,-1) - (3,2,1) = (-2,0,-2)
BC = C-B = (3,2,1) - (1,2,-1) = (-1,-3,1)
CA = A-C= (2,-1,2) - (3,2,1) = (-1,-3,1)
CB X (BC - 2CA)
BC X (BC - 2 (-1,-3,1))
BC X (BC - (-2,-6,2))
BC X ((2,0,2) - (-2,-6,2))
BC X (4,6,0)
(-2,0,-2) X (4,6,0) = (12,-8,-12)
