okay so the original point is (1,4). we know 1 is our X which is the one that is going up. and 4 is our Y which is going to the right. if you are moving 5 units to the right you will add 5 to 1 (our original x) which equals 6. And going 1 unit up our original Y is 4 so we add the 1 and now we get 5. our new point is (6,5)
The function will be where y is the balance, and x is the number of month. The raph will be a line on the xy-plain which starts at the point (0, 360), and end in the point (72, 0). Hope it helps.