Hello everyone !

We are working on a project on Stata where we have to work on this regression : ln(y) = β0 + β1X + β2 ln(population) + γ Geographie
Where "Geographie" contains those three variables : elevat_range_msa, ruggedness_msa and heating_dd which are on our Datalist.
How do I create Geographie ? Hope this question is not too elementary..

Thank you for your help !