The 2 equations are 18.20x+19.50y=230.10 and x+y=12 where x is the months of original cost and y is months for new cost. Since you know that you paid for one year (12 months) you can make the second equation. Then you want to substitute the first equations x by making the second equation x=(12-y) 18.20(12-y)+19.50y=230.10 218.40-18.20y+19.50y=230.10 1.30y=11.70 y=9 so that means you had the original rate for 3 months and the new one for 9 months