If I receive a cash buyout in 2018 as a departing LLC partner at the end of 2017, how do I report the income? I did not receive a K-1 for this received income.

I was one of three partners in a LLC speech pathology practice.  I left the business practice at the end of t2017, and received at the beginning of 2018 a cash payment for 1/3 of the partnership's cash reserves, as a negotiated one-time payment.   I didn't receive a K-1 associated this payment, but a confirmation letter. How do I report this cash payment?