Populate field from 2 dropdown boxes

I have 2 dropdown boxes and one text field.

DropDownList1 has A1 and A2 as choices
DropDownList2 has 5A and 5B as choices

I would like my text field to populate based on the choices.

i.e., if someone picks A1 and 5A from the dropdown boxes, then text field populates as "Planned Development" or if someone picks A1 and 5B then the text field populates as "Subdivision"