a) 35 – 138 + 68 – 35
=35-35+68-138
=0+(-70)
=-70
b) 123 - (-79) + 31 - (31+79)
=123+79+31-31-79
=123+(79-79)+(31-31)
=123 +0+0
=123
c) -21 . 77 + 21 . 20 – 57 . 79
=21 . -77 + 21 . 20 – 57 . 79
=21.(-77+20)-57.79
=21.(-57)-57.79
=-57.(21-79)
=-57.-54
=3306