So the first thing we're going to use is this first property of the circuit that

there's no current flowing into the ideal op-amp.

So if there's no current flowing into this non-inverting input of the op-amp,

that means that Iout is a current which is confined to this loop that we have at

the bottom of our circuit.

So if we want to find Iout, we can use Kirchhoff's voltage law to do that.

And we can use Kirchhoff's voltage law around that mesh.

And in this case, Iout is measured in a counterclockwise direction.

So we're going to sum our voltages up counterclockwise around this circuit.

So if we start at the bottom right-hand corner, we start around the circuit,

we first encounter the negative polarity of the -5 volt source, so it's -5 volts.

And then we have the voltage drop across the 20 kiloohm resistor,

which is 20k times

Iout plus the voltage drop across the 10k as we continue around this loop.

And the voltage across the 10k is going to be 10k times Iout.

And then we return back to the starting point.

And so we know through Kirchhoff's voltage law that

the sum of those voltages is equal to 0.