@tomleslie thanks for the help!! 

Im still a bit confused in how to implement this and i'd be very grateful for any input! My ultimate goal is to get a field plot, but can't because of the unassinged variables (F0, rho0). And which is why i'm trying to give the constants a set of variables. Am I implementing the code that you suggested wrong/would it work in this situation?

Sorry for the consuion and thanks for the response! 

My question is how do I make it so a is equal to set of numbers? is that possible? i.e. a= >0 or a= 1..100 . I can't make a= 1 or a permanent value because it'll mess up calculations later on