3M+6N=3(M+2N)
=3(2x²+4xy-2x-3-2x²+2xy+4)
=3(6xy-2x+1)
=3[2x(3y-1)+1]
3M+6N的值与x无关,所以 2x(3y-1)的值为0,
即 3y-1=0
解得 y=1/3