Standard Addition

[walker8476]

I'm trying to figure out this standard addition equation. I don't get how they arrived at the final answer considering there are two unknowns.

 

[Deleted member 4993]

I'm trying to figure out this standard addition equation. I don't get how they arrived at the final answer considering there are two unknowns.View attachment 2771

Is there a relationship between [Na]_i and [Na]_f ?