-21x^2+77x+28=0

Solution for -21x^2+77x+28=0 equation:

-21x^2+77x+28=0

a = -21; b = 77; c = +28;
Δ = b2-4ac
Δ = 772-4·(-21)·28
Δ = 8281
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{8281}=91$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(77)-91}{2*-21}=\frac{-168}{-42}
=+4
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(77)+91}{2*-21}=\frac{14}{-42}
=-1/3
$