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-4u^2+20u-21=0
a = -4; b = 20; c = -21;
Δ = b2-4ac
Δ = 202-4·(-4)·(-21)
Δ = 64
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:$u_{1}=\frac{-b-\sqrt{\Delta}}{2a}$$u_{2}=\frac{-b+\sqrt{\Delta}}{2a}$$\sqrt{\Delta}=\sqrt{64}=8$$u_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(20)-8}{2*-4}=\frac{-28}{-8} =3+1/2 $$u_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(20)+8}{2*-4}=\frac{-12}{-8} =1+1/2 $
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