You can
put this solution on YOUR website!I recategorised your question so you can get help more quickly and so other people can reference it.
Rate 1 = 72 + 0.18m
Rate 2 = 144 + 0.09m
Set the equations equal and you will get m = 800.
Therefore, you will need to drive at least 800 miles so that Rate 2 is more economical for you.