This screen shows the LINEST() formula being applied to the 2x2 area D4:E5. The cursor is in the function edit box and the areas that the LINEST function is using are outliined in the data area.
If you click on the "fx" box, you will bring up the Function Argument Wizard for filling out the function arguments. When it is active, it will show exaclty what the function requires as arguments and will usually prevent illegal entries. It also is the best entry to a very complete help system for the function. In Excel, click on the "Help on this function" link at the bottom. When you have filled in the arguments, click on the OK button to finish.
Next you MUST press a Shift-Ctrl-Enter key combination to finish entering the array function over the full 2x2 region that is shown in the figure. If successfull, you will see different numbers in each of the four cells. Proceed to the next step.