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Answer:
c = 81
Explanation:
The equation x² -c = 0 can be factored as ...
(x -√c)(x +√c) = 0
which has roots ±√c.
The roots are given as ±9, so we have ...
±9 = ±√c
c = (±9)² = 81
The value c = 81 makes the equation have ±9 as solutions.