-0.1x^2-7.8x+3.9=0

Solution for -0.1x^2-7.8x+3.9=0 equation:

-0.1x^2-7.8x+3.9=0

a = -0.1; b = -7.8; c = +3.9;
Δ = b2-4ac
Δ = -7.82-4·(-0.1)·3.9
Δ = 62.4
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-7.8)-\sqrt{62.4}}{2*-0.1}=\frac{7.8-\sqrt{62.4}}{-0.2}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-7.8)+\sqrt{62.4}}{2*-0.1}=\frac{7.8+\sqrt{62.4}}{-0.2}
$