Solution 12126: Correcting a Non-algebraic Variable in Expression Error on the TI-89 Family and Voyage™ 200 Graphing Calculators.
How can I correct a Non-algebraic variable in expression error on the TI-89 family and Voyage 200 graphing calculators?
A Non-algebraic variable in expression error is returned by a TI-89 family and Voyage 200 if there is a value stored to a variable that appears in the expression. Refer to the steps outlined below in order to restore the variable to its default state (no value stored):
1) Press [CATALOG] then press the [,] key to go to the alphabetical listings beginning with the letter D
2) Scroll down to select the DelVar command and press [ENTER]
3) Input the variable used in the problem (For example, if the variable in the problem is x, then input DelVar x
4) Press [ENTER]
To clear any values stored to all 1-character variables, a-z, refer to the following steps:
1) Press [F6] (On the TI-89 family, press [2nd] [F6])
2) Press [ENTER] or  to choose 1: Clear a-z
To clear any values stored to all 1-character variables, a-z, on the Voyage 200 refer to the following steps:
1) Press [F6] (On the Voyage 200)
2) Press  : Clear a-z...
3) On the diaplay it will read Clear 1-character variables a-z in current folder? press [ENTER] for YES
Now attempt the calculation again.
Please see the TI-89 Titanium / Voyage 200 guidebook for additional information.