3d^2+21d=-30

Solution for 3d^2+21d=-30 equation:

3d^2+21d=-30

We move all terms to the left:

3d^2+21d-(-30)=0

We add all the numbers together, and all the variables

3d^2+21d+30=0

a = 3; b = 21; c = +30;
Δ = b2-4ac
Δ = 212-4·3·30
Δ = 81
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$d_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$d_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{81}=9$
$d_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(21)-9}{2*3}=\frac{-30}{6}
=-5
$
$d_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(21)+9}{2*3}=\frac{-12}{6}
=-2
$