3m + 6n
= 3(m + 2n)
= 3(2x² + 4xy - 2x - 3 -2x² + 2xy + 4)
= 3(6xy - 2x + 1)
= 6x(3y -1) + 3
因为值与x无关
所以3y - 1 = 0
y = 1/3