Step 1 :
After factoring out $ x $ we have:
$$ -x^{3}+x = x ( -x^{2}+1 ) $$Step 2 :
Rewrite $ -x^{2}+1 $ as:
$$ -x^{2}+1 = 1 -x^2 = (1)^2 - (x)^2 $$Now we can apply the difference of squares formula.
$$ I^2 - II^2 = (I - II)(I + II) $$After putting $ I = 1 $ and $ II = x $ , we have:
$$ -x^{2}+1 = (1)^2 - (x)^2 = ( -x+1 ) ( x+1 ) $$