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penfila11Pen
Hey there :)

We know that ( a + b )( a - b ) is a² - b²

So here we apply this formula
( 21 + x )( 21 - x ) = 21² - x²

437 = 21² - x²
437 = 441 - x²
Rearrange the equation
x² = 441 - 437
x² = 4
[tex] \sqrt{x^2} = \sqrt{4} [/tex]

x = +2 or -2

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