The cumulative formation constant for SnCl2(aq) in 1.0M NaNO

The cumulative formation constant for SnCl2(aq) in 1.0M NaNO3is B2= 12. Find the concentration of SnCl2(aq) for a solution in which the concentrations of Sn2 and Cl -are both somehow fixed at 0.31M.