Đáp án:
$\begin{array}{l}
18){x^2} - x - 12\\
= {x^2} - 4x + 3x - 12\\
= \left( {x - 4} \right)\left( {x + 3} \right)\\
19){x^2} + 8x + 15\\
= {x^2} + 3x + 5x + 15\\
= \left( {x + 3} \right)\left( {x + 5} \right)\\
20)3{x^2} - 16x + 5\\
= 3{x^2} - x - 15x + 5\\
= x\left( {3x - 1} \right) - 5\left( {3x - 1} \right)\\
= \left( {x - 5} \right)\left( {3x - 1} \right)\\
21){x^2} - 5x - 24\\
= {x^2} - 8x + 3x - 24\\
= \left( {x - 8} \right)\left( {x + 3} \right)
\end{array}$
