Drive belt for 21"self drive yard king mower

Hi,
Yard King is a Murray model. Since you say it is 21" the model number will begin with that and probably end with "X99" which goes with the name.
Cannot tell you more without model number.
Peace,
Clarke
PS With the model you can look it up here:
http://www.outdoordistributors.com/M...artslists.html