Column Formatting Question

Ok i have a form that has 7 columns in it. if i select something from a dropdown in coulmn 5 how do i get it to automatically put something in column 7 for me....example if i have job numer XX1255 selected in the dropdwon on column 5 and I  want it to automacially populate the name of the job "fiber optic" in column 7 ho do I do that