Regression on the TI-92 (hopefully TI-89 is the same):

HOW TO ENTER DATA:
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Press [APPS].
Choose "Data/Matrix Editor".  
Choose "New" to enter new data ("Current" or "Old" if you've already entered the data).
Choose Type: Data
       Folder: main
       Variable: h1 (you can type any name you want here, it's a reference for bringing up old data later).
Enter your 'x' values in column c1, and your 'y' values in column c2.

HOW TO CALCULATE THE REGRESSION CURVE:
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While your data is still displayed, using the function keys (F1, F2, F3, etc.),
Choose "Calc".
Choose Calculation Type: LinReg (ExpReg and other types of regression are also available)
       x: c1
       y: c2
       Store RegEQ to: y1(x)

HOW TO SET UP YOUR DATA TO BE PLOTTED:
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While your data is still displayed, using the function keys (F1, F2, F3, etc.),
Choose "Plot Setup".
Highlight "Plot 1:". 
Choose "Define".
Choose Plot Type: Scatter
       Mark: Box
       x: c1
       y: c2

HOW TO PLOT YOUR DATA AND THE REGRESSION CURVE:
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Go to your [Y=] window and make sure that "y1=" and "Plot 1" are checked.
Using the function keys (F1, F2, F3, etc.),
Choose "Zoom".
Choose "ZoomData".