-4.9x^2+21x+20=0

Solution for -4.9x^2+21x+20=0 equation:

-4.9x^2+21x+20=0

a = -4.9; b = 21; c = +20;
Δ = b2-4ac
Δ = 212-4·(-4.9)·20
Δ = 833
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{833}=\sqrt{49*17}=\sqrt{49}*\sqrt{17}=7\sqrt{17}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(21)-7\sqrt{17}}{2*-4.9}=\frac{-21-7\sqrt{17}}{-9.8}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(21)+7\sqrt{17}}{2*-4.9}=\frac{-21+7\sqrt{17}}{-9.8}
$