434186 ROI Calculator Revision

I need a slight revision to a project. I will supply you with all files relevant to the ROI calculator. What I have right now is a calculator that has a total of 5 fields that the user fills in, then it solves for ROI. What I need is for the calculator to solve for "X". In other words, the user must be able to fill in any 5 of the 6 fields, and the calculator solves for the 6th field, whatever it is. Simple algebra, the equation is supplied, it just needs a slight revision. I need it in 24 hours.