The monthly payment p on a mortgage varies directly with the amount borrowed b. Suppose that you decide to borrow $120,000 using a 30-year mortgage. You are told that
your monthly payment is $686.26. Assume that you have decided to buy a more
expensive home that requires you to borrow $220,000. What will your monthly payment
be?

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.