There is something stored in the variable x. As a matter of fact, I can tell you it is x=0.000053.
You must clear the variable.
Type in delvar(x) and then hit ENTER to clear x. The screen should then display "Done".
If you want to clear them all, then do this:
Hit F6 and then 'Clear a-z', then ENTER. It should then have a screen that says, "Clear 1-character variables a-z from current folder". Click on ENTER and you're all set. Then try your calc problems.