Which is a factor of 15xy − 45x − 6y + 18?

I see right away that 3 can be factored out of 15xy − 45x − 6y + 18.

The quotient is 5xy - 15x - 2y + 6, so that 

15xy − 45x − 6y + 18   =   3(5xy − 15x − 2y + 6)   (2 factors)

               or                        3 [ 5x(y-1] - 2 [y-3])