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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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