(4x^2+mx-y+n)-(2nx^2-3x+5y-10)
=(4-2n)x^2+(m+3)x-6y+n+10
所以2n=4,n=2,m+3=0,m=-3
则
3(m^2-3mn+5n)-2n^2=3*(9+18+10)-8=111-8=103