Let's break down the problem step by step:

1. Let the unknown number be represented by $x$.
2. According to the problem, when you double the number and subtract 9, you get the original number: $2x - 9 = x$
3. Now, solve for $x$: $2x - 9 = x$ Subtract $x$ from both sides: $2x - x - 9 = x - x$ Simplifying this: $x - 9 = 0$ Now, add 9 to both sides: $x = 9$

So, the number is $\boxed{9}$.

Would you like any further explanation or have more questions about this?

Here are some related questions:

1. How would you solve the equation if the number was tripled instead of doubled?
2. What if instead of subtracting 9, you added 9?
3. How would you represent this problem algebraically if you added a constant instead of subtracting 9?
4. What happens if the number is a negative value?
5. How can you check the solution to this problem by substituting the number back into the original statement?

Tip: When solving equations, always isolate the variable by performing inverse operations to maintain balance on both sides of the equation.