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