1) Large triangle BCD and small triangle DBA.
Corresponding sides are
BC and DB, CD and BA, DB and AD.
Ratios of the corresponding sides should be constant for triangles that are similar.
BC/DB = 18/6 = 3/1
CD/BA =15/5 = 3/1
DB/AD = 6 /2 = 3/1
Triangles BCD and BDA are similar because ratios of the corresponding sides are the same.
2) Δ BCD and Δ DBA
∠DAB of the Δ DBA is corresponding to ∠BDC of the Δ BCD .
∠BDC= 110.5⁰, so ∠DAB = 110.5⁰, because corresponding angles of similar triangles are congruent.