I advise that if you cannot figure out arithmetic sequences by hand, never mind the TI-84. It cannot make you any smarter about these things than you are now. A calculator is a useful tool, but it will not save your quest at the last minute if you do not understand the math you are being tested on.
People have managed to work with arithmetic sequences sans calculators for a very long time. It probably takes longer to figure out how to program a TI-84 to solve problems involving arithmetic sequences than it takes to learn how to solve them by hand.
You need to think for yourself because the calculator cannot think at all. It only does what someone programs it to do. So program your brain a little and see if that helps on your next test.