To use the format, Format>Cells and go to the Number tab and choose the category Custom. In the Type box, delete whatever is there and enter xlhelp's format: #,##0.00;"0";0
Alternatively, skip the format and replace the existing formula with =IF(E6-C6<0,0,E6-C6)
Before you do either, you should decide what you really want in the cell.
If you want it to do math as a zero, use the formula.
If you want it to display a zero but do math as if it were still the negative number, use the format.
If you don't care, use either.
Formats do not change the contents of a cell. They just change how it looks.