If you've got something better than the base package of LabVIEW, there's Eval Formula Node on the functions palette (Mathematics->Formula->Eval Formula Node.vi) or Eval Formula String on the Advanced Formula Parsing sub-palette.
For a little more detail: http://exchange.ni.com/servlet/ProcessRequest?RHIVEID=101&RPAGEID=135&HOID=50650000000800000087810000&UCATEGORY_0=_49_%24_6_&UCATEGORY_S=0