First, let's find 3W:
3W = 3(-3i + 5j - k) = -9i + 15j - 3k
Now, let's add U and 3W:
Zu + 3w = (7i + 5j - 9k) + (-9i + 15j - 3k) = (7 - 9)i + (5 + 15)j + (-9 - 3)k = -2i + 20j - 12k
So, the answer for (a) is $\boxed{-2i + 20j - 12k}$.
(b) To find ||u + w||,
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