(21-43)-(57-36)-(64+21)
=21-43-57+36-64-21
=21+(-43)+(-57)+36+(-64)+(-21)
=[21+(-21)]+[(-43)+(-57)]+[36+(-21)]+(-64)
=0+(-100)+15+(-64)
=-149
*** nhe
(21-43)-(57-36)-(64+21)
=21-43-57+36-64-21
=21-21-43-57-64+36
=(21-21)-(43+57)-(64-36)
=0-100-28
=-128