Solving TI-89 Exact/Approx Homework Problem

[muffintop]

Homework Statement

Recently got a ti-89 and when I use it to derive for example d(x^2+4x-10, x)
I get a decimal, 5.27 instead of getting 2x+4.

The Attempt at a Solution

I tried changing the mode to exact, approx, and auto. All give the same answer.
I tried pressing the diamond button before ENTER, only in the exact mode it gave me a fraction.

 

[Knissp]

Press 2nd, F1 (to get to F6), then "Clear a-z".

Apparently, you have the value of .635 stored as x.

 

[muffintop]

Yay 2x+4!
Haha thanks.