If two triangles have two corresponding side lengths and an included angle to be proportional, then the triangles are similar. This rule is called Side-Angle-Side (SAS) Similarity.
First, lets label the values of each side length
For triangle ABC:
AB=8,AC=17.6
For triangle DEF:
DE=5,DF=11
∠A=∠D as shown by the dots
To prove that ABC is similar to DEF the following equation must be true
ABDE=ACDF
Definition of Similarity
85=17.611
Plug in the side lengths
85=85
Simplify each fraction
Since corresponding side lengths are proportional, and included angles are equal, ABC is similar to DEF