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$21 x^{2} - 8 x - 4 = 0$

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Solution

Given quadratic equation $21 x^{2} - 8 x - 4 = 0$
$x = \frac{- b \pm \sqrt{b^{2} - 4 a c}}{2 a} = \frac{- (8) \pm \sqrt{64 - 4 \times 21 \times (- 4)}}{2 \times 21}$
$= \frac{8 \pm \sqrt{64 + 936}}{42} = \frac{8 \pm 20}{42} = \frac{28}{42}; \frac{- 12}{42}$
$∴ x = \frac{2}{3}, \frac{- 2}{7}$

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