0=-3.9d^2+13.1d+8.7

Solution for 0=-3.9d^2+13.1d+8.7 equation:

0=-3.9d^2+13.1d+8.7

We move all terms to the left:

0-(-3.9d^2+13.1d+8.7)=0

We add all the numbers together, and all the variables

-(-3.9d^2+13.1d+8.7)=0

We get rid of parentheses

3.9d^2-13.1d-8.7=0

a = 3.9; b = -13.1; c = -8.7;
Δ = b2-4ac
Δ = -13.12-4·3.9·(-8.7)
Δ = 307.33
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{-(-13.1)-\sqrt{307.33}}{2*3.9}=\frac{13.1-\sqrt{307.33}}{7.8}
$
$d_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-13.1)+\sqrt{307.33}}{2*3.9}=\frac{13.1+\sqrt{307.33}}{7.8}
$