@rob I still don't see how you have decided that the question asks about a specific physics concept. The amount of effort is not the issue. The question and comments by its author all amount to "I think I have done everything correct but I still don't get the right answer. What is my mistake?"

I did not see the comments before deletion, but this does not clarify the issue for me. I do not see any similarity to a low-effort homework question, this is not the issue. The issue is : is the OP asking about a specific physics concept? The question seems to be asking, "Have I written eqns 1 and 2 correctly?" That is how the accepted answer interprets the question. Is that "asking about a specific physics concept"?

> So, why is equation (1) not applicable in this case?

which seems to me like a question about whether conservation of energy and momentum was the right approach to the problem at all, and if not, why not.