We see that ∠B ≅ ∠E and ∠C ≅ ∠F, therefore, ΔABC ~ ΔDEF by the AA Similarity for Triangles Theorem which states that if two pairs of corresponding angles are congruent in two triangles, then those two triangles are similar. To find the scale factor, all we have to do is DF / AC = 5/2 or 2.5.
