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The roots are 9 and -9

Explanation:
Before we begin, remember the rule of the difference between squares which is as follows:
a² - b² = (a+b)(a-b)

Now, for the given:
x² - 81 = 0
This can be rewritten as:
x² - (9)² = 0
Applying the rule of the difference between squares, we can factor this expression as follows:
(x-9)(x+9) = 0

This means that:
either x-9 = 0 > x = 9
or x+9 = 0 > x = -9

Hope this helps :)