$\begin{array}{l}
12) - 5x\left( {x - 3} \right)\left( {x + 3} \right) + {\left( {x + 1} \right)^3} - {\left( {x - 2} \right)^3} + \left( {x - 1} \right)\left( {{x^2} + x + 1} \right)\\
= - 5x\left( {{x^2} - 9} \right) + \left( {{x^3} + 3{x^2} + 3x + 1} \right) - \left( {{x^3} - 6{x^2} + 12x - 8} \right) + {x^3} - 1\\
= - 4{x^3} + 9{x^2} + 36x + 8
\end{array}$