X2+1.8x-3.9=0

Solution for X2+1.8x-3.9=0 equation:

X2+1.8X-3.9=0

We add all the numbers together, and all the variables

X^2+1.8X-3.9=0

a = 1; b = 1.8; c = -3.9;
Δ = b2-4ac
Δ = 1.82-4·1·(-3.9)
Δ = 18.84
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{-(1.8)-\sqrt{18.84}}{2*1}=\frac{-1.8-\sqrt{18.84}}{2}
$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(1.8)+\sqrt{18.84}}{2*1}=\frac{-1.8+\sqrt{18.84}}{2}
$