Sorry, I glanced at this and massively overcomplicated the problem by suggesting an ICE table (you can use an ICE table, but it is not required). Sorry about that.

For A

B

Keq = [ B]/[ A] (eq. 1)

You know, as Borek already said, that the total concentration [A] + [ B] = 2 mM (eq. 2)

You have 2 unknowns and 2 equations, it's just a simple simultaneous equations matheatical problem. Rearrange one, substitute into the other, solve.

To do it with an ICE table, construct the table as normal:

[A] [ B]

I 2 0

C -x ?

E ? ?

I will let you complete it. Substitute the equilibrium concentrations for [A] and [ B] (i.e. row E) into eq. 1 above and solve.