The triangles are similar by SAS postulate of similarity
Similar triangles are triangles with equal corresponding angles and equal side ratios. This means that the ratio of their corresponding sides are equal.
angle JLN is congruent to angle KLM
=15 /21 = 10/14
= 5/7 = 5/7
This means that the ratio of their corresponding sides is equal and thus the triangles are similar by SAS similarity.
SAS means Side Angle Side postulate of similarity.