a)
A = ( -2a + 3b - 4c ) - ( -2a - 3b - 4c )
= -2a + 3b - 4c + 2a + 3b + 4c
= ( -2a + 2a ) + ( 3b + 3b ) + ( -4c + 4c )
= 0 + 6b + 0
= 6b
b) Thay a = 2012 ; b = -1 ; c = -2013
A = [ -2 . 2012 + 3 . ( -1 ) - 4 . ( -2013 ) ] - [ -2 . 2012 - 3 . ( -1 ) - 4 . ( -2013 ) ]
= 4025 - 4031
= -6