Angle ADB is a right angle, which is given.
The measure of ADB is 90 degrees by definition of right angle.
The two angles are a linear pair (aka they're supplementary) by definition of linear pair (they're formed by two intersecting lines).
The measure of BDC is 90 because BDC + ADB = 180 (definition of linear pair), and ADB = 90 (given).
BDC = ADB because 90 = 90
Overall, the two triangles are congruent by SAS.
