6x^2＋7xy－3y^2－8x＋10y＋c
=(2x+3y+a)(3x-y+b)
=6x^2+7xy-3y^2+(2b+3a)x+(3b-a)y+ab
2b+3a=-8
3b-a=10
解方程组得:b=2,a=-4
所以,c=ab=-4*2=-8