Step 1 :
Rewrite $ x^{4}-6561 $ as:
$$ x^{4}-6561 = (x^{2})^2 - (81)^2 $$Now we can apply the difference of squares formula.
$$ I^2 - II^2 = (I - II)(I + II) $$After putting $ I = x^{2} $ and $ II = 81 $ , we have:
$$ x^{4}-6561 = (x^{2})^2 - (81)^2 = ( x^{2}-81 ) ( x^{2}+81 ) $$Step 2 :
Rewrite $ x^{2}-81 $ as:
$$ x^{2}-81 = (x)^2 - (9)^2 $$Now we can apply the difference of squares formula.
$$ I^2 - II^2 = (I - II)(I + II) $$After putting $ I = x $ and $ II = 9 $ , we have:
$$ x^{2}-81 = (x)^2 - (9)^2 = ( x-9 ) ( x+9 ) $$