设：多项式=（x＋y-2）×（x＋by＋c）=x^2+(b+1)y+by^2+(c-2)x+(c-2b)y-2c
则由对应项相等得：a=b+1;c-2=-5;c-2b=1;-2c=6;得c=-3,b=-2,所以a=-1