Test tomorrow and I'm not gettin' it

HELP!! I have a problem: We're doing geometry applications: circles, cylinders and surface area and I'm being given application problems that I am not understanding.

Solve application problem use 3.14 as the approximate value of (pi) round answers to nearest tenth when applicable.

An irrigation system moves around a center point to water a circular area for crops. If the irrigation system is 50 yd long, how large is the watered area?

this is an odd problem in my book, so I know the answer is 7850 sq yds but I CANNOT seem to figure out how they got to than answer