1. **State the problem:** We have three number cards with values 2, 9, and a question mark (unknown number). The mean of these three cards is 6. 2. **Formula for mean:** The mean of $n$ numbers is given by $$\text{Mean} = \frac{\text{Sum of all numbers}}{n}$$ 3. **Apply the formula:** Here, $n=3$ and the mean is 6, so $$6 = \frac{2 + 9 + x}{3}$$ where $x$ is the unknown number. 4. **Find the total of all three cards:** Multiply both sides by 3: $$3 \times 6 = 2 + 9 + x$$ $$18 = 11 + x$$ 5. **Solve for $x$:** Subtract 11 from both sides: $$18 - 11 = x$$ $$x = 7$$ 6. **Answer:** a) The total of all three cards is 18. b) The number that should replace the question mark is 7.