y^2+0.4y=0.84

Solution for y^2+0.4y=0.84 equation:

y^2+0.4y=0.84

We move all terms to the left:

y^2+0.4y-(0.84)=0

We add all the numbers together, and all the variables

y^2+0.4y-0.84=0

a = 1; b = 0.4; c = -0.84;
Δ = b2-4ac
Δ = 0.42-4·1·(-0.84)
Δ = 3.52
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$y_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$y_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$y_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0.4)-\sqrt{3.52}}{2*1}=\frac{-0.4-\sqrt{3.52}}{2}
$
$y_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0.4)+\sqrt{3.52}}{2*1}=\frac{-0.4+\sqrt{3.52}}{2}
$