Chemical Equilibrium

At equilibrium of the reaction,

\(A_2(g)+B_2(g) \rightleftharpoons 2 A B(g)\)

The concentrations of $A_2, B_2$ and $A B$ respectively are $15 \times 10^{-3} \mathrm{M}, 2.1 \times 10^{-3} \mathrm{M}$, and $1.4 \times 10^{-3} \mathrm{M}$ in a sealed vessel at 800 K . What will be $K_p$ for the decomposition of $A B$ at same temperature ?