A subscript is used to indicate the base of a logarithm. For instance, common logs have a base of 10 while natural logs have a base of e. Unfortunately, the TI-83 graphing calculator does not support subscripts. That, however, does not mean you cannot solve such a log. The key lies in using the "Change of Base Property" to turn such a log into a natural or common log.
Write your log base a of b as common log b divided by common log a: log_a(b) = log(b)/log(a). For example, log_2(100) would turn into log(100)/log(2).
Submit the first log by pressing the "LOG" button, entering the value of b, pressing the right parenthesis key and pressing the division key.
Finish the calculation by pressing the "LOG" button, entering the value of a, pressing the right parenthesis key and pressing the "ENTER" button.