6400=-16t+800t^2

Solution for 6400=-16t+800t^2 equation:

6400=-16t+800t^2

We move all terms to the left:

6400-(-16t+800t^2)=0

We get rid of parentheses

-800t^2+16t+6400=0

a = -800; b = 16; c = +6400;
Δ = b2-4ac
Δ = 162-4·(-800)·6400
Δ = 20480256
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{20480256}=\sqrt{2304*8889}=\sqrt{2304}*\sqrt{8889}=48\sqrt{8889}$
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(16)-48\sqrt{8889}}{2*-800}=\frac{-16-48\sqrt{8889}}{-1600}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(16)+48\sqrt{8889}}{2*-800}=\frac{-16+48\sqrt{8889}}{-1600}
$