x^3 - 2x^2 -5x + 6= 0
<=> x^3 - x^2 - x^2 + x - 6x +6=0
<=> x^2 (x-1) - x(x-1) - 6(x-1) =0
<=> (x-1) (x^2 -x - 6) = 0
<=> (x-1) (x^2 +2x - 3x -6) = 0
<=> (x-1) ( x(x+2) - 3(x+2)) =0
<=> (x-1) (x+2) (x-3) = 0
th1 x-1=0 th2 x+2=0 th3 x-3=0
x=1 hay x= -2 hay x=3
=>