Write the expression in complete factored form

(x + 6)(x - y)

We can see that 6x - 6y have a common factor of 6 in the same way that x^2 - xy have a common factor of x.
So, when we group our factors, we get:
x^2 - xy + 6x - 6y = x(x - y) + 6(x - y) = (x + 6)(x - y)