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They're going to look at everything that still appears on your credit report, regardless of how far back it is. The last two years can provide a 'death blow' to the application, so if the report is clean for 2 years back, you're in pretty good shape. However, that doesn't mean they're not interested in explanations for stuff that happened longer ago.
I would provide a general explanation, whatever that might be, but make sure you also paint it in the light of why that won't happen now. Maybe your income is significantly higher. They basically want to have an idea of what happened, what you did to resolve it and why you won't repeat the mistake.
Good luck.