Let's solve the expression $3^n - 3^{n-1}$.

We can factor the expression to simplify it:

$3^n - 3^{n-1} = 3^{n-1}(3 - 1) = 3^{n-1} \cdot 2$

So, the simplified form of the expression is:

$2 \cdot 3^{n-1}$

Would you like further details or have any questions about this? Here are some related questions:

1. How do you factor expressions involving exponents?
2. What are the properties of exponents?
3. Can you apply this method to expressions with different bases?
4. How would this simplification change if the base were different?
5. What happens if $n = 1$ in this expression?

Tip: Remember that factoring is a useful technique when simplifying expressions with powers of the same base.