Determine the moment about the origin O of the force (3 lb) (6 lb) (F= i− j+4 lb) k which acts at a point A. Assume that the position vector of A is
(a) r = −(7.5 ft)i + (3 ft)j − (6 ft)k,
(b) r = −(0.75 ft)i + (1.5 ft)j − (1 ft)k, (c) r = − (8 ft)I +(2 ft)j − (14 ft)k.