2A-B
=2(x²-ax-1)-(2x²+3ax-2x-1)
=2x²-2ax-2-2x²-3ax+2x+1
=-5ax+2x-1
=(-5a+2)x-1
因为多项式2A-B的值与字母x的取值无关
所以-5a+2=0
解得a=2/5
答案：a=2/5