由（x-y）^7=x^7-7x^6y+21x^5y2-35x^4y3+35x3y^4-21x2y^5+7xy^6-y^7x+2y中的x与（-35x^4y3)相乘,系数是-352y与（21x^5y2)相乘，系数是42，所以-35+42=7
