6a-(8-a)+(5a-12)

To simplify the expression \( 6a - (8 - a) + (5a - 12) \), follow these steps:

1. Distribute the negative sign in front of the first parentheses: \[ 6a - 8 + a + 5a - 12 \]

2. Combine like terms: \[ 6a + a + 5a - 8 - 12 \] This simplifies to: \[ (6a + a + 5a) + (-8 - 12) \] which is: \[ 12a - 20 \]

So the simplified expression is: \[ \boxed{12a - 20} \]