6.
a)
[tex]12 - ( - x + 6) = 12 + x - 6 = x + 6[/tex]
b)
[tex](c + b) - (2c - b) - (c - 2b) = c + b - 2c + b - c + 2b = 4b - 2c[/tex]
c)
[tex](2ab + b + 3) - (3ab - b + 3) = 2ab + b + 3 - 3ab + b - 3 = - ab + 2b[/tex]
7.
[tex]12(x - 1) + 3(x - 2) = 12x - 12 + 3x - 6 = 15x - 18[/tex]