I just purchased this calculator tonight and have been going over the
basic algebraic commands.  I can master them all it appears but one,
once I set a variable the TI-89 just does not forget the value of it
for any problem you work.

The example question I've been using is: 3x^2+5x-7 when x=-1.

I then proceed to enter this into the TI-89:

     -1[STO->]X:3x^2+5x-7

It gives me the correct answer -9, but every problem I work with an x
variable it still gives it that original value of -1.

So finally here is my question, first off, am I doing the question
right on the TI-89 in the first place, or should I use the solve(
function?  (And if I should could you go step by step of how it's
entered into the TI-89 using the solve( function.)

Or do I somehow "clear" the value of x in the calculators memory? 
(And if you do somehow "clear" it would you walk me through step by
step.)

Thanks!