First, we need to find the Cayley tables for U(7), D3, and U(9).
(a) U(7) is the group of units modulo 7, which consists of the elements {1, 2, 3, 4, 5, 6}. The operation is multiplication modulo 7. The Cayley table for U(7) is:
$$
\begin{array}{c|cccccc}
\times
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