Two ways come to mind:
1. Use an { EQ \o (6,D) } field. To do this, type the field text shown
between the curly braces (but not the curly braces themselves), manually
subscript the 6 and superscript the D, highlight the field text, press
Ctrl+F9, and press Alt+F9 twice.
2. Insert an Equation Editor object. To do this, click on Insert | Object
| Microsoft Equation 3.0. There is a ready-made equation format for what you
want to do here.