Writing superscripts in Excel:

 The process is not quite as easy in Excel as it is in Word.  Type the formula or descriptor that contains the superscript as though it does not have a superscript.  Example:  To write degrees Celsius start by typing oC. Then, using your mouse pointer, wipe across the o in oC to highlight it (it will look like this: oC).  Click Format, Cells, click the Font tab at the top, and check the word superscript.  Click the OK button and your symbols should now look like this:  ^oC.

Extrapolating using a graphing:

Enter your data in the appropriate columns in Excel.  Example: 

 Highlight those cells.  Click on the Chart Wizard icon (its the one that looks like a tiny bar chart).  Choose the XY (scatter) graph, and click next button.  On the next dialogue box, click the title tab and set up your title, and axes labels.  Click the gridlines tab and check all the boxes.   Click on the Legends tab and uncheck Show legend.  Click next and check New sheet, and click the Finish button. 

 To set up the trend line, click the word Chart on the top toolbar.  Click the Add trendline option. Click on the type of trendline that you need for this data (most of the time it will require a linear trendline).  Click the options tab and type in 300 under Forecast: and next to Backward (this will extrapolate your trendline to a point that should be far enough to show where it crosses the x axis).  Check the display equation on chart option and click the OK button.

 If you cant see your dots very well, you can make them larger by clicking on them and increasing the size.

 You can make the minor gridlines darker and the minor gridlines lighter and therefore the whole graph easier to understand by double clicking on those gridlines and choosing those options that you think might help.  If it doesnt help, then just click the Undo button and try something else.   

Determining the value of the extrapolation predicted by your graph:

You can do this visually by determining the point where the trendline crosses the line of interest (if you are predicting the value of absolute zero using the kinetic theory of gases, you will look to see where the trendline crosses the x axis).  You can do this mathematically by using the equation given you by Excel.  Example:  If the equation is y = 0.6121x + 169.74 and I want to know the value of x where the trendline crosses the x axis, well y = 0 on the x axis, so I change y to 0 and use algebra to find x (which is the temperature) [if were trying to find the value of absolute zero that happens where y (which is the volume) is equal to 0].   So,  

y = 0.6121x + 169.74

Þ 0 = 0.6121x + 169.74

Þ -(169.74/0.6121) = x

Þ -277.30762947230844633229864401242 = x

 Þ -277.3 = x

and since x is Temperature (in ^oC), absolute zero is predicted to be -277.3^oC.