Solve each equation by factoring. Simplify all irrational and complex solutions.

9. x^3+216=0

10. 24x^3 –81=0

11. x^3 = x^2+20x

12. x ^4+13x^2+40=0

13. x^4-x^2= x^2+8

14. 3x^4-8x^2+5=0

15. 4x^3+5x^2-16x-20=0