This polynomial equation below, I solved using TI-82 and I only got one root:
\(\displaystyle 4x^3-16000x^2+16*10^6x-3072*10^3=0\)
so got the first root as 250. How do I find the other ones, or use this root to find out other ones?

Hawk, your link has the $x^0$ coefficient as 3 billion, not 3 million. Also, the root was only approximately 251. It seems nitpicky, but in some cases a small error in the root in your denominator can produce large errors in the subsequent roots.