Answer:
x = 7 or -6
Explanation:
Let a number be x.
Its square = x²
x - x² = 42
-x² + x - 42 = 0
-(x² - x + 42) = 0
x² - x + 42 = 0
(x - 7)(x + 6) = 0
x - 7 = 0 or x + 6 = 0
x = 7 or x = -6
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