Proof by Cases

Prove by cases:

1) Statement: If ? ∈ ℤ, then ?

? + ?? + ? is an odd integer.

Proof:

2) Statement: If ? ∈ ℤ, then ?
? − ? is even.

Prove:

3) Prove the following statement by Proof by Contrapositive, and Proof by Contradiction:
Statement: If ?? − ? is odd, then ?? + ? is even