Answer:
SAS similarity theorem
Explanation:
The diagram shows two triangles with congruent angles A and A'.
Consider the ratio of given sides:
As you can see, the ratio is the same.
SAS similarity theorem states that if two sides in one triangle are proportional to two sides in another triangle and the included angle in both are congruent, then the two triangles are similar.
Hence, triangles ABC and A'B'C' are similar by SAS similarity theorem.