So let's start with our knowledge of op-amps and see where it leads.

So we know that this voltage at the non-inverting input of the op-amp is 0

volts because it's tied to ground.

We know that the voltage at the inverting input is the same as the non-inverting

input so the voltage at this node is also 0 volts.

And so that gets us a long ways toward solving the problem.

It allows us to then take advantage of Kirchhoff's current law and

nodal analysis to write an equation around this node for the currents that

are flowing into it or out of it depending our choice on which one we use.

So if we use Kirchhoff's current law and we call this node one and

we're going to use Kirchhoff's Current Law on node one and

we're going to sum the currents that are flowing into node one.

The current through the resistor flowing left to

right is going to be V sub n minus 0 divided by R.