I am a bit confused about how to set up this problem:

A cow is tied to a silo with radius 6 by a rope just long enough to reach the opposite side of the silo. Find the area available for grazing by the cow. The rope must be 6pi in length, but I am not sure what to do with it from there.

Solving via polar equation seems simpler to me, but I need to know how to solve it with parametric equations. Any help would be appreciated. Thanks.