If we know that Angle L is congruent to Angle O, then we can use the AAS (angle angle side) congruence property. We have one pair of angles marked by the square marker (angle J and angle M). So they are congruent angles. We have a pair of congruent sides JK = MN = 3. So we're just missing a pair of angles.
Note: The answer is NOT angle K = angle N because this would mean ASA would be used instead of AAS. The order of the letters is important as it establishes how the sides and angles relate. With ASA, the side is between the angles. With AAS, the side is not between the angles.