You mean subtracting 2 values from each other in FME or creating formulas and write those to Excel?

If it's formulas you want, take a look at this post. If you want to subtract 2 values from each other, there's a lot of options. I usually use an AttributeCreator (or AttributeManager nowadays) and then open the Arithmetic Editor. Double-click on the values you want to subtract and type a '-' in between.