D=-4.9t^2+30t

Solution for D=-4.9t^2+30t equation:

=-4.9D^2+30D

We move all terms to the left:

-(-4.9D^2+30D)=0

We get rid of parentheses

4.9D^2-30D=0

a = 4.9; b = -30; c = 0;
Δ = b2-4ac
Δ = -302-4·4.9·0
Δ = 900
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$D_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$D_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{900}=30$
$D_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-30)-30}{2*4.9}=\frac{0}{9.8}
=0
$
$D_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-30)+30}{2*4.9}=\frac{60}{9.8}
=6+1/8.16666666667
$