These are all #3xx2# matrices and are therefore compatible for the multiplication and addition that is asked.
#A=[(5,7),(-1,6), (3,-9)], B=[(8,3), (5,1), (4,4)], C=[(0,4),(-2,5), (7,-1)], D[(6,2), (9,0), (-3,0)]#,
#3C=3[(0,4),(-2,5), (7,-1)] = [(0,12),(-6,15), (21,-3)]#
#-4A = -4[(5,7),(-1,6), (3,-9)] = [(-20,-28),(4,-24), (-12,36)]#
#3C -4A +B = [(0,12),(-6,15), (21,-3)] + [(-20,-28),(4,-24), (-12,36)]+[(8,3), (5,1), (4,4)]#
#=[(-12 ,-13), (3,-8), (13,37)]#