My book suggests that x = mass FeO (or mol FeO).
I like the look of this
Personally, I would let x = mole of FeO. These kind of problems can be done very systematically, are there no examples in your textbook?
For example, your second problem, this is how I would do it:
m(NaCl) + m(KCl) = 4.000 g
n(NaCl) + n(KCl) = n(AgCl)
You can easily calculate n(AgCl). Now we just need to manipulate the first equation slightly using the relationship between moles (n), mass (m) and molar mass (M). We can substitute mass with moles x molar mass, so we end up with:
58.45 x n(NaCl) + 74.55 x n(KCl) = 4.000 g
You now have two equations with two variables, so you can solve to find moles of NaCl and KCl. The first problem can be done the same way.