Why cant 81-x to the forth power be factored unti (3+x) squared times (3-x) squared

Below.

Step-by-step explanation:

81 - x^4 Treat as the difference of 2 squares:

= (9 - x^2) (9 + x^2)

We can further factor the 9 - x^2 (difference of 2 squares)

= (3 - x) (3 + x) (9 + x^2).

9 + x^2 can't be factored.