Because two of the corresponding angles are equal, the two triangles are similar based on the AA similarity theorem
What is the AA similarity theorem?
The AA (Angle-Angle) similarity theorem asserts that when two angles in one triangle are congruent to two angles in a second triangle, the two triangles are considered similar. Consequently, the corresponding angles within each triangle establish the similarity between the two triangles.
In the image shown, we see that angles R and G are equal to each other and angles S and F are equal also. Therefore, triangles RSQ and GFH are similar triangles based on the AA similarity theorem.