$a,{x^3} - 5{x^2}y - 14x{y^2}$
$= x\left( {{x^2} - 5xy - 14{y^2}} \right)$
$= x\left( {{x^2} - 7xy + 2xy - 14{y^2}} \right)$
$= x\left( {x - 7y} \right)\left( {x + 2y} \right)$
$b,{x^2} + 8x + 7$
$= {x^2} + x + 7x + 7$
$= \left( {x + 1} \right)\left( {x + 7} \right)$
Học tốt !