There is no formula!!! If you understand what it means then it is common sense. It is the difference in sales (at standard contribution) resulting from the change in the share of the total market.
Don’t simply learn formulae for the exam – the examiner deliberately asks questions that test that you understand what is happening rather than have just learned formulae. That is why only 50% of the exam involves calculations – the rest is again testing understanding.
If a specific question is causing you problems on this, then state which question and I will explain the answer.