How to properly use unknown variables in an equation?

I'm sure this is pretty simple but for some reason I can't make sense of the help files.  I'm not sure what the proper terminology is so here's an example of what I'm trying to do: 

I would like to have Mathcad tell me that f = .055P  (i.e. P/18).  How do I do that?  Right now it tells me that P is undefined; do I just need to define it, and if so, how do I do that?

Thanks in advance!