Question

what is the first statement of the proof? ∠3 and ∠4 are vertical angles ∠3≅∠4 given ∠1 and ∠3 are a linear pair

Explanation:

In a proof, the first - step is usually stating what is given. Here, the most appropriate first statement would be something that is provided as a starting - point. "∠3 and ∠4 are vertical angles" is a given fact which can start the proof.

Answer:

∠3 and ∠4 are vertical angles