I have a collection on my credit after bankrupcy and i want to buy a house

MBrown it would depend on the chapter of Bankruptcy you filed, 7 or 13, how you have handled credit obligations since then, have you reestablished a positive credit profile and the type/size of the collection. Bankruptcy should be a clean slate, a do over. So any negative credit after the Bankruptcy will have a significant impact on your credit score and application. You can try to provide the letter of explanation, but my guess is if the Underwriter would consider the letter they would have requested it. You may want to try contacting another lender, ask co-workers, family, friends for a recommendation.