An expression is shown below:
3x^3y + 12xy - 9x^2y - 36y
factor this completely

3x³y + 12xy + 9x²y - 36y
3y(x³) + 3y(4x) + 3y(3x²) - 3y(12)
3y(x³ + 4x + 3x² - 12)

So the first thing we do is to multiply the top equation by 3. the reason i want to do this is i notice there will be a 6y on both equations and can possibly add/subtract them. so we get 6y = -3x + 27. adding that and the second equation we get 6x = 27-15 which is 12. divide by 6 and we get 2. ∴ x = 2. substituting 2 into the first equation we get 2y = -2 + 9 which is 7. divide by 2 and we get 7/2. ∴ y = 7/2, x = 2.