Nomenclature of an Alkane
According to IUPAC (source):


  2.6 - If chains of equal length are competing for selection as main chain in a saturated branched acyclic hydrocarbon, then the choice goes in series to:
  (a) The chain which has the greatest number of side chains.


Therefore the correct name is 3-ethyl-2,2-dimethylhexane as opposed to 3-tert-butylhexane because the former has more side chains.