Did I get it right ?
The totla Credit was 1000
The annual interestrate is 7%
What is a "backinterest"?, the monthly payment (including interest) to payback the credit?
Assumung that is so:

The formula to compute the remaining credit after any month is:
Credit= (Credit* 7%/12)-400

Do that until Credit is less than 0. I know it'S an iterative way, but EXCEL is fast enough to do that.