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7X^2-14X-21=0
a = 7; b = -14; c = -21;
Δ = b2-4ac
Δ = -142-4·7·(-21)
Δ = 784
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}$$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}$$\sqrt{\Delta}=\sqrt{784}=28$$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-14)-28}{2*7}=\frac{-14}{14} =-1 $$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-14)+28}{2*7}=\frac{42}{14} =3 $
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