Pls help, due in 4 minutes

Ineed i do not understand i'm only 8

What is the missing constant term in the perfect square that starts with x^2 -20x

Fiona races bmx around a circular dirt course. if the radius of the course is 70 meters, what is the total distance fiona covers in two laps of the race? a. about 30,772 meters b. about 15,386 meters c. about 879.2 meters d. about 439.6 meters e. about 219.8 meters23 points hurry