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Step-by-step explanation:
The correct order of the steps is ...
- (Given)
- (Definition of supplementary)
- (Substitution)
- (Subtraction ...)
- (Definition of congruent angles)
A proof always starts with "given". It always ends with a statement of what you have proved.
To fill in the sequence between these, it helps to think about what the relationships are and why you can conclude that the theorem is correct.