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7z^2-21z-28=0
a = 7; b = -21; c = -28;
Δ = b2-4ac
Δ = -212-4·7·(-28)
Δ = 1225
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{1225}=35$$z_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-21)-35}{2*7}=\frac{-14}{14} =-1 $$z_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-21)+35}{2*7}=\frac{56}{14} =4 $
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