Also, there's a real subscript 2 at Unicode position 2082, which is
included in several fonts that come with Windows or Office (such as
Arial, Lucida, Cambria, Calibri, and other of the "C" fonts from
Office2007), but unfortunately not in Times New Roman.
It's designed so it doesn't look grayed out when used in formulas like
this, and you don't need to bother with subscripting.
To access it, type (using the regular keyboard) 2082, then Alt-X.
If you need it a lot, you can assign a keyboard shortcut to it via
Insert Symbol -- go to the "Superscripts and Subscripts" range of
Insert Symbol, select it, and click the Shortcut button.