The answer is use the perpendicular bisectors to find the center of the circle.
Answer:
Some people advise that in very cold weather, you should keep the gas tank in your car more than half full. Irene's car had 6.1 gallons in ... in your car more than half full. Irene's car had 6.1 gallons in the 15 gallon tank on the coldest day of the year. Irene filled the tank with gas that costs $3.80 per gallon.
Hope this picture helps! ;) Ignore the stuff around it ($12 or the triangle)