For. a, we know that Troy's truck can hold about 30 gallons at most. For each gallon he can travel an average of 21. They tell us to write an equation to represent the gas Troy has after driving X amount of miles.
X - Would be the amount of miles driven
Y - total amount of gas in the truck
y=-21x+630
The reason the equation is like this is because the total amount is equal to 630 miles (the amount the truck can hold with 30 gallons of gas) minus the amount of miles he travels. The reason its 630 and not 30 is because we are given "21 miles" so we must convert the gas to miles.
For B, your teacher says that theres only 15 gallons of gas in Troy's truck. So we know that each gallon of gas is 21 miles, and if he already used up 15 gallons, we can do 15x21 which is 315 miles. So for B it would be 315 miles since he last filled up his gas tank.
If you don't understand anything be sure to tell me.