I have attached 3 screenshots to show you how to use the build-in functions of the TI89.
1. The text of the program.
2. How to start the program. (probier is the German word for trial)
3. The IO-screen
The command Input requires a numerical expression. I named the variable for the first expression e, for the second expression f. Even though I asked for an equation you put in a term. The terms e and f are stored in y1(x) and y2(x).
The Disp command shows the result of solving the equation e = f.
I hope this helps a little bit. If you need some additional informations: ask.