105=0.8d^2-2.4d

Solution for 105=0.8d^2-2.4d equation:

105=0.8d^2-2.4d

We move all terms to the left:

105-(0.8d^2-2.4d)=0

We get rid of parentheses

-0.8d^2+2.4d+105=0

a = -0.8; b = 2.4; c = +105;
Δ = b2-4ac
Δ = 2.42-4·(-0.8)·105
Δ = 341.76
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$d_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$d_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$d_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(2.4)-\sqrt{341.76}}{2*-0.8}=\frac{-2.4-\sqrt{341.76}}{-1.6}
$
$d_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(2.4)+\sqrt{341.76}}{2*-0.8}=\frac{-2.4+\sqrt{341.76}}{-1.6}
$