a) -(12+21-23)-(23-21+10)
=-12-21+23-23+21-10
=(-21+21)+(23-23)-12-10
=-12-10
=-22
b) 26.(-125)-125.(-36)
=(-125).(26-36)
=(-125).(-10)
=1250
c) 33.(17-5)-17.(33-5)
=33.17-33.5-17.33+17.5
=(33.17-17.33)+(-33.5+17.5)
=-33.5+17.5
=5.(-33+17)
=5.(-16)
=-80