Angle ADB is a right angle --- Given
Angle ADB is 90 degrees --- Definition of a Right Angle
Angle CDB is 90 degrees --- Definition of Supplementary Angles
Angle ADB = Angle CDB --- Definition of Congruent Angles
Proofs can be tricky! What helps me is thinking through them step by step and how I would prove I did something. While I'm not entirely sure what options you have for this proof, I've given my best guess.
Hope this helps!! :)
