EXPLANATION
Let's see the facts:
Number of miles driven = 720
Traveled time = 9 hours
Let's call r to the rate, y to the number of miles driven and x to the hours, the appropiate equation is:

Then, as x=9 and y=720, replacing terms:

Dividing both sides by 9:

Simplifying terms:

Switching sides:

Answer: I was traveling at 80 mph