0=-3.9t^2+15.9t

Solution for 0=-3.9t^2+15.9t equation:

0=-3.9t^2+15.9t

We move all terms to the left:

0-(-3.9t^2+15.9t)=0

We add all the numbers together, and all the variables

-(-3.9t^2+15.9t)=0

We get rid of parentheses

3.9t^2-15.9t=0

a = 3.9; b = -15.9; c = 0;
Δ = b2-4ac
Δ = -15.92-4·3.9·0
Δ = 252.81
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{-(-15.9)-\sqrt{252.81}}{2*3.9}=\frac{15.9-\sqrt{252.81}}{7.8}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-15.9)+\sqrt{252.81}}{2*3.9}=\frac{15.9+\sqrt{252.81}}{7.8}
$