Diagnose why your answer was wrong

I got a problem wrong and want to understand why, not just see the solution.

Subject: {{subject}}. Level: {{level}}.
The problem: {{problem}}
My answer and working: {{attempt}}
The correct answer (if I have it): {{correct}}

Please:
1. Walk through my working line by line and point to the first place it went wrong.
2. Name the underlying misunderstanding (concept, rule, careless slip or misread question) in one sentence, with the reason I might have made it.
3. Teach the missing idea briefly using one simple example that is different from this problem.
4. Show the correct solution in clear steps.
5. Give me two new practice problems targeting the same weak point, one easy and one harder, with answers hidden at the end.
6. Give me a quick rule, check or habit to catch this mistake next time.

If my working is ambiguous, tell me what you assumed. If the "correct answer" I gave appears to be wrong, say so. If you are unsure about a fact, say so plainly. Never invent citations, quotes, statistics or URLs.