To find the measure of angle \( mJKN \), we need to first analyze the given information about the angles and the parallel lines \( JL \) and \( MO \).
Since \( JL \) and \( MO \) are parallel lines, we can use the properties of angles formed when a transversal intersects parallel lines.
Given that \( mONP = 50° \):
- The angle \( ONP \) forms a pair of alternate interior angles with angle \( JKN \) because \( JN \) can be seen as the transversal intersecting the lines \( JL \) and \( MO \).
- Since the lines are parallel, we have: \[ mJKN = mONP \] which means: \[ mJKN = 50° \]
Thus, the measurement of angle \( mJKN \) is \( 50° \).