I am not sure it is valid. Consider the case when all variables except u are true and u is false.

And in general, what does one mean by "establish the validity of the argument"? There are so many ways to do it, from truth tables to formal proofs, and it completely depends on what instructor's plans for the course are. How were similar exercises solved in the past?

In fact, everybody knows what "and", "or", "not" and "implies" mean. So, even without knowing much technical details, you can try to come up with informal argument that either validates or invalidates the conclusion. Wrapping this informal argument into a form required by the course is often completely straightforward.