Desolve with symbolic constants

[Gene222]

Check to see if a user variable "a" was created. On the CAS screen, press the Vars key, tap the User tab, then tap User Variables. A list of user variables will appear. If there is a variable a, use the arrors keys to highlight the variable, and the press the backspace or correction key to delete the variable.

This might make desolve(y''=y/a,x,y) work on the CAS screen. As previously stated, use single quotes. As near as I can tell for your problem, there should not be any cas or user variables for x,y, or a.