a: =>3x-2x+6=6
=>x+6=6
=>x=0
b: =>4(2x-1)-12(x+1)=3(x+2)
=>8x-4-12x-12=3x+6
=>-4x-16=3x+6
=>-7x=22
=>x=-22/7
c: =>(x-1)^2-(3x+3)^2=0
=>(x-1-3x-3)(x-1+3x+3)=0
=>(-2x-4)(4x+2)=0
=>x=-1/2 hoặc x=-2
d: =>x^2-3x-4+x^2+3x-4=2(x^2-1)
=>2x^2-8=2x^2-2
=>0x=6(loại)