Use the definition of a Midpoint and the definition of congruent polygons.
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Since L is the midpoint of JN and KM, by definition we have that JL≅ LN and ML≅ LK.
We are told that JM≅ NK. Then, the three sides of △ MJL are congruent to the three sides of △ KNL, and so we can apply the Side-Side-Side (SSS) Congruence Postulate.
Finally, since △ MJL ≅ △ KNL, by the definition of congruent polygons all the corresponding parts are congruent. This means that ∠ MJL ≅ ∠ KNL.
