a)-5 ( 3x - 7 ) - ( -15x +3 ) - ( 12-x )= -4
=>-15x+35+15x-3-12+x=-4
=>(-15x+15x)+(35-3-12)+x=-4
=>0+20+x=-4
=>20+x=-4
=>x=-4-20
=>x=-24
b)-3 ( 4x -2 ) - ( -12x +8 ) - (-x)=0
=>-12x+6+12x-8+x=0
=>(-12x+12x)+(6-8)+x=0
=>0+(-2)+x=0
=>(-2)+x=0
=>x=0-(-2)
=>x=2