How would you find the Merkle Root with a Block that contains 6 transactions?
You would duplicate the hash of the last two txs. The tree would "look" something like this:

             abcdefef
              /  \            
           /        \          
        /              \       
      abcd             efef     
      /  \            /  \    
   /        \      /        \  
  ab        cd    ef        ef 
 / \        / \   / \  
a   b      c  d  e   f