Rachel leaves her house on her bike. she rides at a constant speed until she reaches a lemonade stand, where she parks her bike and takes a rest. then she turns around and bikes home as fast as she can.

Use stokes' theorem to evaluate c f · dr where c is oriented counterclockwise as viewed from above. f(x, y, z) = xyi + 5zj + 7yk, c is the curve of intersection of the plane x + z = 8 and the cylinder x2 + y2 = 81.