Your equation would be y = 2.75x + 4
The equation format I used was y = mx + b. 'Y' is the total, and 'X' is the number of buckets. Because the amount of $2.75 payments depends on how many buckets you use, that is 'M', or the rate of change. 'B' is represented by $4 because that is a one time payment made.
Hope this helps!