Advice on clearing an old joint loan.

Hi. 20 years ago my wife and myself consolidated owed money with a unsecured joint loan to the sum of 24k. After 4 years we'd paid off about 8k. We got divorced, me and our 3 daughters went our own way and for the past 14 years they've been raised by me. My ex said she'd pay off the debt (this was only a verbal contract). I've received no financial help from her during the last 14 years btw. The other day a letter was posted to me by a debt agency asking for 14k as she's only paid off 2k in the last 14 years. Does anybody have knowledge of how i should manage this situation?