Question 907622
You might want y to represent cost of renting the lawnmower.


Let y be cost
Let x be days of rent


The $4 is included with the days at x=1, but would be not included for x=0, or zero days of rent.  The flat fee would still be used, but in practice, a person would not be charged any such $18 if he not renting any lawnmower.


At x=0, y=18; and for each 1 day increase, the $4 per day is applied, which becomes the slope of the equation; the 18 is the y-intercept.


{{{highlight(y=4x+18)}}}