David ran one half of a mile down the high school track and then walked one third of a mile

i got 10.28 but that rounds up to 10.3

check above link.

graphically?   well, you graph them both, the solutions is where they both intersect each other, since there are two solutions, meaning they will meet twice, or namely the linear will run into the quadratic curve twice, and where they meet, those x,y coordinates are the solutions to the system.