I realize this must be a very simple question, but how do I evaluate expressions containing symbols?

Suppose you have a simple expression, for instance:

X*(M-1)I would like to press on a key that did:

-------

X*(M+1)

- Prompts for all the values of the symbols, such as X, M in the example, showing the previous value if any.

- Returns the value of the expression.

Many thanks... as you can see, I am a beginner with my 50g.

Luca