a) a - (a - (2a - 4))

To simplify the expression \( a - (a - (2a - 4)) \), follow these steps:

1. Start from the innermost parentheses:

   \[ 2a - 4 \]

2. Substitute this back into the expression:

   \[ a - (a - (2a - 4)) = a - (a - (2a - 4)) \]

   This simplifies to:

   \[ a - (a - 2a + 4) \]

3. Now simplify the expression inside the parentheses:

   \[ a - (a - 2a + 4) = a - (-a + 4) = a + a - 4 \]

4. Combine like terms:

   \[ 2a - 4 \]

So, the simplified expression is:

\[ \boxed{2a - 4} \]