You used the if operator incorrectly. The format is
if( condition, what to do if True, what to do if False)
But you put another condition in the place of "what to do if false".
Here's an example: I want a function which is equal to $3x$ between $x=1$ and $x=4$, is equal to $x+8$ between $x=4$ and $x=5$, and is undefined otherwise. The code could be
if(x>5, ln(0), if(x>4, x+8, if(x>1, 3*x, ln(0) ) ) )
Here I'm using $\ln(0)$ with the hope that it will simply produce no output for the corresponding range, rather than halt the entire computation. Don't have the app (or iPad for that matter), so can't tell for sure.
This is only possible in a document, not in the calculator application itself.
Create a new document and add the calulator app (press 1).
Now open the menu (menu button) and select "Functions and Programs" (press 9), open the program editor (press 1) and create a new file (press 1).
Name your first function, in your case that would be y. Change the type to "Function" and click OK.
You are now presented with this text
Define y()=
Func
EndFunc
You can now write the function definition between "Func" and "EndFunc" and have it output with the "Return" statement. In your case you would write
Return w+2
Save and check your function with Ctrl+B. You can now use it in your calculator app, which should be empty for now. If you write now y() it will return w+2.
But if you save a value to w, in your case $3→w$, and write y() again, it will return 5.
For x() you can do the same and since you want to use y(), you can define the function like this to access y():
Return 2*y()
Writing x() should now return 10.
As a side note: You have to write the brackets after x and y, because the brackets indicate that a function has to be called, while x and y without brackets would indicate that they are stored values. Also you can't use the normal calulator app, since all the functions in there are pure functions and can't access stored variables. You have to stay within the document.
Best Answer
To plot $x=f(y)$ without having to mentally flip across the $y=x$ diagonal, you could plot $y = -f(x)$, then turn the calculator 90° counterclockwise. In your case, plot $y=-(x^2-6)$.