I'm not too sure about what you mean by over-substituting,

well, i do know what you mean, but are my equations over-substituted?

i mean, there are two variables and i have two equations

I know that those equations can be used to solve the question,

I've been told that they will work...

I took a look at the equation you suggested,

it tells me that [S^{2-}]=2[Ag^{+}],

which is equal to the solubility of silver sulfide