Help with this math problem?

The fuel efficiency (mpg rating) for cars has been increasing steadily since 1980. The formula for a car's fuel efficiency for a given year between 1980 and 1996 is

E=0.36x+15.9

where E is miles per gallon and x is the number of years after 1980.

a. In what years was the average fuel efficiency for cars less than 17 mpg (in interval form)?

b. In what years was the average fuel efficiency for cars more than 20 mpg (in interval form)?

thank you!