3D Trig. Question

ABCD (in clockwise direction) is a rectangular piece of paper with AB = 6 and BC = 12. M and N are 2 points on AD and BC respectively such that AM = BN = 4. The piece of paper is then folded along MN making an angle of 75.5 degree between plane ABNM and MNCD. Find the angle between line BD and plane ABNM. (Note : BD is calculated to be 10 after folding.) (Ans. 50.8 degree)