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z^2+20z-21=0
a = 1; b = 20; c = -21;
Δ = b2-4ac
Δ = 202-4·1·(-21)
Δ = 484
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:$z_{1}=\frac{-b-\sqrt{\Delta}}{2a}$$z_{2}=\frac{-b+\sqrt{\Delta}}{2a}$$\sqrt{\Delta}=\sqrt{484}=22$$z_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(20)-22}{2*1}=\frac{-42}{2} =-21 $$z_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(20)+22}{2*1}=\frac{2}{2} =1 $
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