Points (1) and (2) that unplebeian
mentions are often coined, the golden rules
of op-amps and may be applied to these kinds of questions.
Using, the rule: input to an op-amp draws (essentially) no current, you can easily deduce what happens to Rs.
You can use the expression that unplebeian describes in point (3). However your own expression for Vo is equally valid and will agree in solution.