答：
3m^2n^2+12mn+12+x(mn+2)^2
=3(m^2n^2+4mn+4)+x(mn+2)^2
=3(mn+2)^2+x(mn+2)^2
=(3+x)(mn+2)^2
3x^2-2x+1/3
=1/3*(9x^2-6x+1)
=1/3*(3x-1)^2