Rewrite $ y^{2}-289 $ as:
$$ y^{2}-289 = (y)^2 - (17)^2 $$Now we can apply the difference of squares formula.
$$ I^2 - II^2 = (I - II)(I + II) $$After putting $ I = y $ and $ II = 17 $ , we have:
$$ y^{2}-289 = (y)^2 - (17)^2 = ( y-17 ) ( y+17 ) $$