Need help beginning a modular congruence proof

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(∀a, b ∈ Z) a2 + b2 − 3 ≡|≡ 0 mod 4. Hi, I'm not too sure where to start with this proof and just need some pointers on how to approach this.

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a=b=3 gives 9+9-3 = 15 which is congruent to 0 mod 3, disproving your statement.