5а-(6а-(7а-(8а-9)))

Let's simplify the expression step by step:

1. Start with the expression: \[ 5a - (6a - (7a - (8a - 9))) \]

2. Begin by simplifying the innermost parentheses: \[ 8a - 9 \] This remains as \(8a - 9\).

3. Now substitute this back into the expression: \[ 7a - (8a - 9) \] Distributing the negative sign: \[ 7a - 8a + 9 = -a + 9 \]

4. Substitute this result into the next set of parentheses: \[ 6a - (-a + 9) \] Again distribute the negative sign: \[ 6a + a - 9 = 7a - 9 \]

5. Now substitute this back into the original expression: \[ 5a - (7a - 9) \] Distributing the negative sign: \[ 5a - 7a + 9 = -2a + 9 \]

So, the simplified expression is: \[ \boxed{-2a + 9} \]