a+2b=5x²+2x-1+2(-3x²-x+5)
=5x²+2x-1-6x²-2x+10
=-x²+9

错了吧：a=5x²+2x²-1还是5x²+2x-1已知a=5x²+2x²-1,b=-3x²-x+5，a+2b=奥a+2b=5x²+2x²-1+2(-3x²-x+5)=5x²+2x²-1-6x²-2x+10=x²-2x+9