The postulate or theorem that proves each pair of triangles similar is Side-Angle-Side similarity theorem.
[tex]\begin{gathered} \text{Tan }\angle M=\frac{15}{21}=\frac{5}{7} \\ \text{Tan }\angle L=\frac{15+5}{28}=\frac{20}{28}=\frac{5}{7} \end{gathered}[/tex]
Since angle M and angle L are acute, then
[tex]\angle M\cong\angle L[/tex]
