Stephen bought 7 bottles of ketchup and 3 bottles of mustard for $22. rebekah bought 3 bottles of ketchup and 1 bottles of mustard for $8 write a system of equations

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.