Quantifiers, valid arguments and De Morgan
Negate "all", draw "every multiple of 6 is a multiple of 3", tell valid arguments from invalid ones and use De Morgan's laws on sets.
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Guide
"All" and "some" are quantifiers. The negation of "all A are B" is "at least one A is not B", and one counterexample is enough to show an "all" statement false.
An argument is valid when its conclusion has to be true whenever its facts are true. From "if p then q" and p you may conclude q, and from "if p then q" and not q you may conclude not p. You may not go from q to p, or from not p to not q.
De Morgan's laws: the complement of a union is the intersection of the complements, (A∪B)′ = A′∩B′, and (A∩B)′ = A′∪B′. "Not" turns "and" into "or" and "or" into "and".