256=512t-16t^2

Solution for 256=512t-16t^2 equation:

256=512t-16t^2

We move all terms to the left:

256-(512t-16t^2)=0

We get rid of parentheses

16t^2-512t+256=0

a = 16; b = -512; c = +256;
Δ = b2-4ac
Δ = -5122-4·16·256
Δ = 245760
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:

$\sqrt{\Delta}=\sqrt{245760}=\sqrt{16384*15}=\sqrt{16384}*\sqrt{15}=128\sqrt{15}$
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-512)-128\sqrt{15}}{2*16}=\frac{512-128\sqrt{15}}{32}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-512)+128\sqrt{15}}{2*16}=\frac{512+128\sqrt{15}}{32}
$