a=-3 b = 1过程？2x^2+ax-y+b-(2bx^2-3x+5y-1)=(2-2b)x^2+(a+3)x-y+b-5y+1要不含x^2和x需2-2b=0a+3 = 0所以a=-3 b = 12-2b怎样得的？2x^2+ax-y+b-(2bx^2-3x+5y-1)=2x^2+ax-y+b-2bx^2+3x-5y+1(把后面的括号打开就可以了)=(2-2b)x^2+(a+3)x-y+b-5y+1