The same way you factor expressions with one variable:
>factor(12x^2-17x*y+6y^2)
(3*x - 2*y)*(4*x - 3*y)Just make sure that you write x*y and not xy. The first is the product of x and y, but the second is the single variable named "xy." The factor() function can be found in the Algebra menu (F2->2 on the home screen).