Ervin sells vintage cars. every three months, he manages to sell 13 cars. assuming he sells cars at a constant rate, what is the slope of the line that represents this relationship if time in months is along the x-axis and the number of cars sold is along the y-axis?

What we know:

x-axis is months and we can set it up as increments of 3 months since he sells 13 cars every three months

y-axis is cars sold and we can set it up as increments of 13

slope=rise/run 
At 0 months we sold 0 cars and at 3 months we sold 13 cars. So, from point (0,0) we can do rise over run to reach the point (3,13). We rise 13 and run right 3. 

slope=rise/run=13/3

(3,13) is the simplest slope then increases to (6,26), (9,39) And continues.