-7x^2-2x+28=0

Solution for -7x^2-2x+28=0 equation:

-7x^2-2x+28=0

a = -7; b = -2; c = +28;
Δ = b2-4ac
Δ = -22-4·(-7)·28
Δ = 788
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:

$\sqrt{\Delta}=\sqrt{788}=\sqrt{4*197}=\sqrt{4}*\sqrt{197}=2\sqrt{197}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-2)-2\sqrt{197}}{2*-7}=\frac{2-2\sqrt{197}}{-14}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-2)+2\sqrt{197}}{2*-7}=\frac{2+2\sqrt{197}}{-14}
$