1.
Principle of electroneutrality. Less dispersal of "formal" charges........
a. +4 -3
b. +2 -1 b is favored.
There is also priority order you should take consider. I believe other guys will help you regarding this issue soon. If I remember correctly, less dispersal of formal charges is in priority.
2.
The sum of formal charges must equal to the charge of that atom or ions.