OM = 18, ZNMK = ?

In right triangle ONM, ON = 9, OM = 18. Thus, sin(ZMON) = ON/OM = 9/18 = 1/2, so ZMON = 30 degrees. Since ON is perpendicular to NM, ZONM = 90 degrees. In triangle ONM, ZNMO = 180 - 90 - 30 = 60 degrees. Since NK is tangent to the circle at N, ON is perpendicular to NK, so ZONK = 90 degrees. ZNMK = ZONK - ZNMO = 90 - 60 = 30 degrees.