t4-81 is a difference of 2 squares and can be written as (t2-9)(t2+9)
t2+9 can't be further factorised but t2-9 is a difference of 2 squares again and can be factorised to (t+3)(t-3)
so the factors of t4-81 are :(t2+9)(t+3)(t-3)
Hope this helps :-)
I believe the answer you are looking for is (t - 3)(t + 3)(t 2 + 9)
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