1. **State the problem:** Simplify the expression $5x^2 - 13x - 8 + 9x$. 2. **Combine like terms:** The terms involving $x$ are $-13x$ and $+9x$. 3. **Perform the addition:** $-13x + 9x = -4x$. 4. **Rewrite the expression:** Now the expression is $5x^2 - 4x - 8$. 5. **Final simplified expression:** $$5x^2 - 4x - 8$$ This is the simplified form of the given expression.