creating a formula using the results of another formula

creating a formula using the results of another formula

Hi - 
Newbie here again. This seems very basic:

I'm trying to create a series of calculations. Many of them depend on the values returned from other formulas.

How do I accomplish this?

Those formulas don't appear as options in the Field list when creating my formula.

I mean, that on a form I want to have a formula that calculates x+y (resulting in z)
Then I want another field on that page to calculate  = (z+ another value). But it don't show z the formula field