Step 1 :
Rewrite $ a^4-y^4 $ as:
$$ \color{blue}{ a^4-y^4 = (a^2)^2 - (y^2)^2 } $$Now we can apply the difference of squares formula.
$$ I^2 - II^2 = (I - II)(I + II) $$After putting $ I = a^2 $ and $ II = y^2 $ , we have:
$$ a^4-y^4 = (a^2)^2 - (y^2)^2 = ( a^2-y^2 ) ( a^2+y^2 ) $$Step 2 :
Rewrite $ a^2-y^2 $ as:
$$ \color{blue}{ a^2-y^2 = (a)^2 - (y)^2 } $$Now we can apply the difference of squares formula.
$$ I^2 - II^2 = (I - II)(I + II) $$After putting $ I = a $ and $ II = y $ , we have:
$$ a^2-y^2 = (a)^2 - (y)^2 = ( a-y ) ( a+y ) $$