Can you give us an example of what you're trying to do? The Arduino can do math at runtime, and the IDE (compiler) pre-processor can do some math too.
You'll need to write an expression with the "answer" on the left like this:
X = 1 + 2;
Or,
X = Y +3; // Where Y has already been defined.
And, you can do things that don't make sense in "regular math" like:
X = X + 1;