Again, chemistry help!!!!!!!!!!

Question

What is the selenide ion concentration [Se2-] for a 0.100 M H2Se solution that has the stepwise dissociation constants of Ka1 = 1.3 × 10-4 and Ka2 = 1.0 × 10-11?

a. 3.6 × 10-3 M

b. 1.3 × 10-5 M

c. 1.3 × 10-4 M

d. 1.0 × 10-11 M

Answer 1

1.3 x 10^-4 = x^2 / 0.100-x

x = [H+] = [HSe-] = 0.00360 M

HSe- <-----> H+ + Se2-

1.0 x 10^-11 = ( x + 0.00360)(x) / 0.00360-x

x = 1.0 x 10^-11 M = [Se2-]