This is a very short proof. It requires 3 lines only. Two of which are already given by the teacher. The given statements are simply repeats of what the instructions provide.
The last line will be the statement
and the reason is "transitive property". The last line of all proofs is what we're trying to prove.
The transitive property is where we have A = B and B = C, so it must mean A = C. Think of a set of dominoes where A leads to B then to C. In short, A must lead to C.
A similar idea is the substitution property. We replace one thing for another as long as the two things are equal.