d^2=-15d

Solution for d^2=-15d equation:

d^2=-15d

We move all terms to the left:

d^2-(-15d)=0

We get rid of parentheses

d^2+15d=0

a = 1; b = 15; c = 0;
Δ = b2-4ac
Δ = 152-4·1·0
Δ = 225
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$d_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$d_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{225}=15$
$d_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(15)-15}{2*1}=\frac{-30}{2}
=-15
$
$d_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(15)+15}{2*1}=\frac{0}{2}
=0
$