The roots for your cubic are 3, 6, and 8. I got that from here:
Soving Cubic Equations
Your discriminant was less than 0, which means the cubic has 3 real unique roots.
Since you want to divide by x - 8, you can use the synthetic division calculator I posted in the computer software forum also located here:
Dividing your cubic by (x - 8), I get
That's a quadratic equation that you can easily solve for your last 2 roots.
Check out the math for the synthetic division to see where you went astray. Based on how close you and I are, I bet it was your last step in the constant column for synthetic division.
Let me know if you have questions.