18=-18d-d^2

Solution for 18=-18d-d^2 equation:

18=-18d-d^2

We move all terms to the left:

18-(-18d-d^2)=0

We get rid of parentheses

d^2+18d+18=0

a = 1; b = 18; c = +18;
Δ = b2-4ac
Δ = 182-4·1·18
Δ = 252
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$d_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$d_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{252}=\sqrt{36*7}=\sqrt{36}*\sqrt{7}=6\sqrt{7}$
$d_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(18)-6\sqrt{7}}{2*1}=\frac{-18-6\sqrt{7}}{2}
$
$d_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(18)+6\sqrt{7}}{2*1}=\frac{-18+6\sqrt{7}}{2}
$