They mature exactly 10 years from today. The yield to maturity is 12%, so the bonds now sell below par. What is the current market value of the firm's debt?

Here's a couple of threads where I've gone over the basics of this. Give them a read and then give it a shot.

https://www.askmehelpdesk.com/finance/how-do-you-compute-current-price-bonds-200150.html
https://www.askmehelpdesk.com/finance-accounting/computing-bond-price-199941.html