To simplify the expression $- (2a - 3b) - (-a + 3b)$, let's distribute the negative signs inside the parentheses:

1. Distribute the negative signs: $- (2a - 3b) = -2a + 3b$ $- (-a + 3b) = a - 3b$

2. Now, substitute these into the original expression: $-2a + 3b + a - 3b$

3. Combine like terms: $(-2a + a) + (3b - 3b)$

   This simplifies to: $-a + 0 = -a$

So, the simplified expression is:

$\boxed{-a}$

Would you like further details or have any questions?

Here are 5 related questions to explore:

1. How do you simplify expressions with multiple sets of parentheses?
2. What are like terms, and why do they matter in algebraic expressions?
3. How do you distribute negative signs when simplifying expressions?
4. What are the rules for combining terms in polynomial expressions?
5. How would the simplification change if there were coefficients on $a$ or $b$?

Tip: Always pay close attention to the signs (positive or negative) when simplifying expressions to avoid errors.