3! You have to ensure balance of all the different elements.
Positive ion with a radius smaller than the radius of the atom
Answer: I think It might be 1 M???
Explanation: Sorry I'm not in high school I put the wrong age

Here's the balanced equation for given Double displacement reaction ~

The products fored are : Lead Iodide ( PbI2 ) and Potassium Nitrate ( KNO3 )