Rewrite $ y^{2}-100 $ as:
$$ y^{2}-100 = (y)^2 - (10)^2 $$Now we can apply the difference of squares formula.
$$ I^2 - II^2 = (I - II)(I + II) $$After putting $ I = y $ and $ II = 10 $ , we have:
$$ y^{2}-100 = (y)^2 - (10)^2 = ( y-10 ) ( y+10 ) $$