There are VIs that come with LabVIEW (probably full / pro only) that let you do this, provided I understand what you're asking.
I suggest searching the palettes for "Eval Formula String.vi". This is the "easy" way to do it, but it will parse your formula string every time it is called. This parsing can be rather time consuming. If you want to do the parsing first, and then run the calculation multiple times, you can split up the parsing and evaluation by using "Parse Formula String.vi" first, then use the outputs from it to a call to "Eval Parsed Formula String.vi" inside a loop.
