Solve by:
$ab=0 \rightarrow $ : $a=0 $ ou $b=0$:

Got it !

Solve:

$x^2-9 = 0$ $(x+3)(x-3) = 0 $ so: $S=\{-3;3\}$
$x^2-3x=0 $
$x^2+10x+21=0 $
$x^2+2x=-1 $
$(x-9)^2=1 $
$x^3+x^2=4x+4 $
$x^5=2x^4+x-2 $