To simplify the given expression, we need to follow the order of operations, often remembered by the acronym PEMDAS (Parentheses, Exponents, Multiplication and Division (from left to right), Addition and Subtraction (from left to right)). First, simplify the expressions inside the innermost parentheses, then proceed outward.
Step 1: Simplify the Innermost Parentheses
First, we simplify the expression inside the innermost parentheses:
\[
-7 + 6 = -1
\]
Now, the expression becomes:
\[
-9 + [(5 - 4) - (-1)]
\]
Step 2: Simplify the Next Parentheses
Next, we simplify \(5 - 4\):
\[
5 - 4 = 1
\]
Now, substituting this back into the expression gives us:
\[
-9 + [1 - (-1)]
\]
Step 3: Final Simplification
Now, we simplify \(1 - (-1)\):
\[
1 - (-1) = 1 + 1 = 2
\]
Thus, the expression simplifies to:
\[
-9 + 2 = -7
\]