c+c*c-0.4=0

Solution for c+c*c-0.4=0 equation:

c+c*c-0.4=0

Wy multiply elements

c^2+c-0.4=0

a = 1; b = 1; c = -0.4;
Δ = b2-4ac
Δ = 12-4·1·(-0.4)
Δ = 2.6
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$c_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$c_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$c_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(1)-\sqrt{2.6}}{2*1}=\frac{-1-\sqrt{2.6}}{2}
$
$c_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(1)+\sqrt{2.6}}{2*1}=\frac{-1+\sqrt{2.6}}{2}
$