-（3m^2-mn+n^2）-(4m^2-2mn-n^2)
=-3m^2+mn-n^2-4m^2+2mn+n^2
=-7m^2+3mn
m=-1/2,n=-1/3
所以
原式=-7*1/4+3*(-1/2)*(-1/3)
=-7/4+3/2
=-1/4