a.
- The independent value is $3.75.
- The dependent value is $1.25 because it depends on the kilometer ride.
- Let us set x for each kilometer.
- Let us set F for the toral fare
Then, we can write the next equation that relates the fare to the distance.

b.
If we want to find the total cost for a 21-km ride. Then, we need to set x=21 on the equation.
Hence,

Hence, a 21-km ride has a total cost of $30.