7x^2+3y+xy+21x
=(7x^2+21x)+(3y+xy)
=7x(x+3)+y(x+3)
=(7x+y)(x+3)
2ac-6ad+bc-3bd
=(2ac-6ad)+(bc-3bd)
=2a(c-3d) +b(c-3d)
=(2a+b)(c-3d)