分解因式7x^2-3y+xy-21x
问题描述：
分解因式7x^2-3y+xy-21x
答
分解因式7x^2-3y+xy-21x=7x(x-3)+y(x-3)=(x-3)(7x+y)
答
7x^2-3y+xy-21x
=7x^2+xy-3y-21x
=x(7x+y)-3(y+7x)
=(7x+y)(x-3)