设原多项式为M
根据题意得:M+(x²-7x-8)=2x²-x+5
所以:M=(2x²-x+5)-(x²-7x-8)
=x²+6x+13
所以:M-(x²-7x-8)
=(x²+6x+13)-(x²-7x-8)
=13x+21