Homework question on chemically defined medium
A chemically defined medium is, according to this Wikipedia article:   


  ... a growth medium suitable for the in vitro cell culture of human or
  animal cells in which all of the chemical components are known.  


Four of the five components fit this criterion precisely, as they are chemical formulae (and of course a common name for a compound whose formula is well known).  

However, even though you know exactly how much yeast extract was added to the medium, you do not know its exact chemical composition. To be sure, assays of the chemical composition of yeast extract have been run, and one could come up with approximations of it's makeup. But the bottom line is that there is no precise chemical formula for yeast extract as there are for the other components of the medium.