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