The slope (or average rate of change) formula is:

. It is easy to remember as "slope is easy and fun, rise over run" where the y-values of the 2 points would be the "rise" and the x-values of the 2 points would be the "run" So with two points, you label one point as

and the other as

. Then you insert these points into the slope formula (m represents slope). So,

where (-3,6) is x2 and y2 and (5,1) is x1 and y1. It would simplify to

.
Answer:
40 months
Step-by-step explanation:
Since it takes him 2 months to drive 15,000 miles
He drives 7,500 miles/month (15,000 miles / 2months)
Dividing the total distance by his speed yields the time it takes to drive that distance
300,000 miles / 7,500 miles/month = 40 months
Three hundred and twenty seven point seven nine