We require the Ksp value for BaCO3 to solve this equation.
The Ksp for BaCO3 is 5.1 x 10-9
The reaction can be given as :
Ba(NO3)2 + CaCO3 = BaCO3 + NaNO3
So, precipitation of Ba2+ will start at 5.1 x 10-5 M concentration.
Hope it helps.