This says there are 6 elements in \(A \cap B\text{,}\) 4 elements in \(A \setminus B\) and 2 elements in \(B \setminus A\text{.}\) Now these three sets are disjoint, so we can use the additive principle to find the number of elements in \(A \cup B\text{.}\) It is \(6 + 4 + 2 = 12\text{.}\)
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