Solution :
Give that,
Todd had 6 gallons of gasoline in his motorbike.After driving 150 miles, he had 3 gallons left.
When Todd had 6 gallons of gasoline in his motorbike he drived 0 miles,
And When he had 3 gallons left , he had drived 150 miles.
Slope or rate of change = 
Slope or rate of change of gasoline is negative which means gasoline is decreasing with increase in miles.
Slope or rate of change of gasoline= - 0.02 gallons/mile