The number 21 X09 is equal to 22 000, correct to 2 significant figures. Find the value of X if 21 X09 is a perfect square. I lucky guessed the answer is 6 but I need to know how thanks​

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.