Look at the effects of growth hormone (gibberellic acid)
on plant height with a 0.1 molar solution: 
Acid Plant height in one week 
20 ml 18.5 cm
30 ml 20.7 cm
40 ml 45.2 cm
50 ml 62.3 cm
60 ml 10.6 cm 
What is the estimated plant height at 10 ml of acid?
What is the estimated plant height at 25 ml of acid?

With your TI-83 or 84 graphing calculator,

First, make sure the Diagnostics are on.  To make sure,
 
PRESS 2nd 0 to get CATALOG
Press and hold the down arrow key till pointer gets to 
DiagnosticOn

(If you go too far use the up arrow to get the pointer
 back to it)

Press ENTER ENTER

You should read 

DiagnosticOn
            Done
Press CLEAR

---------------------------

Make sure Plot1 is on.

Press Y= to get STAT PLOT then up arrow to highlight 
Plot1 then ENTER

Press 2nd MODE to get Quit (to go back to main screen)

---------------------------

Make sure TABLE SETUP is on ASK for the independent 
variable

Press 2nd WINDOW to get TBLSET

Press down arrow twice and right arrow once to 
highlight Ask

Press 2nd MODE to get Quit (to go back to main screen)

-------------------------

Press STAT ENTER

Enter the values 20, 30, ..., 60 in L1
Enter the values 18.5, 20.7, .... 10.6 in L2

Press STAT, then right arrow to highlight CALC

Scroll with down arrow to highlight LinReg(ax+b) and
press ENTER 

Press VARS, then right arrow to highlght Y-VARS, then 
press ENTER ENTER

Read
LinReg
 y=ax+b
 a=.258
 b=21.14
 r²=.0358107396
 r=.1892372573

That value or r² is not near 1, so CLEAR.

-------------------------------------------

Press STAT again, then right arrow to highlight 
CALC

Scroll with down arrow this time to highlight 
QuadReg and press ENTER

Press VARS, then right arrow to highlight Y-VARS,
then press ENTER ENTER

You should read
QuadReg
 y=ax²+bx+c
 a=-.0822857143
 b=6.840857143
 c=-94.06 
 r²=.5457879872

That value or r² is also not near 1, so CLEAR.

-----------------------------------------------

Press STAT again, then right arrow to highlight 
CALC

Scroll with down arrow this time to highlight 
CubicReg and press ENTER 

Press VARS, then right arrow to highlight Y-VARS,
then press ENTER ENTER ENTER

Read
CubicReg
 y=ax³+bx²+cx+d
 a=-.0075916667
 b=.8287142857
 c=-27.01797619
 d=288.56 
 r²=.9922768519

Now that value of r² is very near 1 so we have found a
good regression function for the data.  However, let's
try the quartic regression to see if its r² value is
even closer to 1. If it's not closer then we'll come 
back to this one.

-----------------------------------------------

Press STAT again, then right arrow to highlight 
CALC

Scroll with down arrow this time to highlight 
QuartReg and press ENTER 

Press VARS, then right arrow to highlight Y-VARS,
then press ENTER ENTER ENTER

Read
QuartReg
 y=ax4+bx³+cx²+dx+e
 a=-1.320833E-4
 b=.0135416667
 c=-.3807916667
 d=2.115633333 
|e=41.3
 
Press the down arrow to read 
 R²=1

Now that value of R² is exactly 1, so we have found 
the best possible standard regression function for 
the data. 

To see the data points plotted and watch the graph go
through them, press ZOOM, then scroll down to highlight 
ZoomStat  
Then press ENTER

---------------------------

Now to get the answer you want, press 2nd GRAPH to 
get TABLE

Enter 10 under the X column, press ENTER, 
read 36.6 under Y1 column

Enter 25 under the X column, press ENTER, 
read 16.195 under the Y1 column

Edwin