the sum of the interior angles of a polygon is (n-2)*180, n being the number of sides.
a nonagon has nine sides, so the total of interior angles is (9-2)*180=7*180
this is a regular nonagon, so each interior angle has the same degrees:
7*180/9=7*20=140
the exterior angle=180-the interior angle=180-140=40
40 is the correct answer.