0=-4.9t^2+28.7t

Solution for 0=-4.9t^2+28.7t equation:

0=-4.9t^2+28.7t

We move all terms to the left:

0-(-4.9t^2+28.7t)=0

We add all the numbers together, and all the variables

-(-4.9t^2+28.7t)=0

We get rid of parentheses

4.9t^2-28.7t=0

a = 4.9; b = -28.7; c = 0;
Δ = b2-4ac
Δ = -28.72-4·4.9·0
Δ = 823.69
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{-(-28.7)-\sqrt{823.69}}{2*4.9}=\frac{28.7-\sqrt{823.69}}{9.8}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-28.7)+\sqrt{823.69}}{2*4.9}=\frac{28.7+\sqrt{823.69}}{9.8}
$