Rick has d dollars. The formula for the number of gallons of gasoline that Rick can buy with d dollars is given by g = d/3. The formula for the numbers of miles that Rick can drive on g gallons of gasoline is given by m = 21g. How many miles can Rick drive on $8 worth of gasoline?

G, the number of gallons, is a function of d, the dollars. The rule is:

g (d) = d/3. For example, with 30 dollars, Rick can buy g (30) = 30/3=10 gallons.

the number of miles that Rick can travel, m, is a function of g, described as follows:

m (g) = 21g

for example, with 30 gallons, Rick can travel m (30) = 21*30=630 miles.

now, with 8 dollars, Rick can buy g (8) = 8/3 gallons of gasoline,

with 8/3 gallons, Rick can travel m (8/3) = 21 * (8/3) = 7*8=56 miles

Answer: 56 miles.