a) \( x^{3}+9 x^{2}-3 x+5-7 x^{3}+9 x+8= \)

To simplify the expression, first combine like terms. Specifically, combine the \( x^3 \) and \( x \) terms: \[ x^{3} - 7x^{3} = -6x^{3} \] Next, combine the \( x^2 \) term: \[ 9x^{2} \] Now for the linear \( x \) terms: \[ -3x + 9x = 6x \] Finally, combine the constant terms: \[ 5 + 8 = 13 \] So the simplified expression is: \[ -6x^{3} + 9x^{2} + 6x + 13 \]