Logic problems

Oh gosh, I am struggling with these three problems. Please help in any way you can! It would be greatly appreciated.

1. Do a proof of the following valid argument:
∀x (Medium(x) ∧ Cube(x))
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∃z Medium(z)


2. Do a proof:
∀x (Large(x) ∧ ¬Dodec(x))
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∀y ¬Dodec(y)


3. Do a proof:
∀x (Tet(x) → Large(x))
∃z ¬Large(z)
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∃x ¬Tet(x)