Does your TI-84 calculator have the "logbase" function? Take a moment to check. Turn your calculator on and press the "math" key. Hit "^" once or twice to scroll up the list. If you have the "logbase" function it will be item A or B in the list. If you choose it, you will see "log" on your screen with a space to type in the base of the logarithm and another space to type in the number you wish to take the logarithm of.
If you with to take log to the base 3 of 81, you could press "math", choose the "logbase" function, type in "3", press ">" to tab over and then type "81" to get the correct answer "4."
Whether or not your calculator has this function seems to depend more on how new it is than on which model you have. It's a convenient function to have if you are taking the ACT or the SAT II Math 2 subject test, so take a moment to get out your calculator and look.