Given the following [H3O+] or [OH-] and the concentration of the solution calculate the dissociation constant, Ka.

[OH-] = 3.9 x 10^(-9) M, with a concentration of 0.50M

Answer:1.3 x 10^(-11)

How do I get this answer?

Not a very well worded question but here is how you do it.

.........HA ==> H^+ + A^-
I.......0.5.....0......0
C.......-x......x......x
E......0.5-x....x......x
(H^+) = x

The problem tells you OH^- = 3.9
E-9. Use (H^+)(OH^-) = Kw = 1E-14, substitute OH and Kw and solve for H^+. Then substitute into Ka = (H^+)(A^-)/(HA) and solve for Ka. I get 1.3E-11.