Because MK is a diameter, then angle L is a right angle. We already know that the measure of angle K is 50, so the measure of angle M has to be 40 because of the triangle angle-sum theorem. The rule for inscribed angles and the arcs they cut off is that the angle is half the measure of its intercepted arc or, likewise, the arc is twice the measure of the angle that cuts it off. Since arc LK is across from angle M and is cut off by angle M, then arc LK is twice the measure of angle M, and is 80. That's the same reason why angle L is 90; arc MK is a semi-circle, with a degree measure of 180, and angle L is half of that.
