100=0.25d^2

Solution for 100=0.25d^2 equation:

100=0.25d^2

We move all terms to the left:

100-(0.25d^2)=0

We get rid of parentheses

-0.25d^2+100=0

a = -0.25; b = 0; c = +100;
Δ = b2-4ac
Δ = 02-4·(-0.25)·100
Δ = 100
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$d_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$d_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{100}=10$
$d_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-10}{2*-0.25}=\frac{-10}{-0.5}
=+20
$
$d_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+10}{2*-0.25}=\frac{10}{-0.5}
=-20
$