0=4.9*t^2+8.6*t-30

Solution for 0=4.9*t^2+8.6*t-30 equation:

0=4.9t^2+8.6t-30

We move all terms to the left:

0-(4.9t^2+8.6t-30)=0

We add all the numbers together, and all the variables

-(4.9t^2+8.6t-30)=0

We get rid of parentheses

-4.9t^2-8.6t+30=0

a = -4.9; b = -8.6; c = +30;
Δ = b2-4ac
Δ = -8.62-4·(-4.9)·30
Δ = 661.96
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-8.6)-\sqrt{661.96}}{2*-4.9}=\frac{8.6-\sqrt{661.96}}{-9.8}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-8.6)+\sqrt{661.96}}{2*-4.9}=\frac{8.6+\sqrt{661.96}}{-9.8}
$