Question

Let g(x) be the number of gallons of gas used by a car during a drive, where x is the number of miles. Interpret the following:
g(20)=0.8

g(130)=5.6

Answer 1

well, they have said that
Let g(x) be the number of gallons of gas used by a car during a drive, where x is the number of miles.

So, replacing x by 20,
Let g(20) be the number of gallons of gas used by a car during a drive of 20 miles. That is, it takes 0.8 gallons to drive 20 miles.

similarly for the other one.