Write a situation that could be represented by the equation y=15x-3

A tree grows at a rate of 15 inches per year.  However, three inches were chewed off by quasi-sadistic bunnies the first year it was planted.  Fortunately, the bunnies were frightened off by the garden gnomes, and did not return.  As a result the tree was able to grow unencumbered for the rest of its life.