SAS Similarity Theorem
If two sides of one triangle are proportional to two sides of another triangle and the included angle in both are congruent, then the triangles are similar by the SAS theorem.
We need to check if the conditions are met in the triangles given in the question.
First, let's test the proportionality of the sides.
In triangle ABC, side AB has a measure of 9 units
In triangle DEF, side DE has a measure of 6 units.
The proportion is 9/6 = 1.5. This is the scale factor.
Now check the other given sides.
In triangle ABC, side CA has a measure of 6 units
In triangle DEF, side FD has a measure of 4 units.
Proportion is 6/4 = 1.5
Given the scale factor is identical for both triangles, the first condition is met.
Now we can see the included angles BAC and EDF are congruent because they have the same measure of 40°.
Since both conditions are met, we conclude the triangles are similar by the SAS theorem
