Answer
$(x - 3) (x+3)(x^2 +9)$
Work Step by Step
$x^4 - 81 = (x^2)^2 - (9)^2 = (x^2 - 9) (x^2 +9) = ((x)^2 - 3^2)(x^2 + 9) = (x - 3) (x+3)(x^2 +9)$
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