The topic of this problem is parallel and series resistors. In this problem, we have a resistor network and we want to find the equivalent resistance RAB for the resistor network. RAB is measured at the left-most side of the circuit and the circuit contains this parallel and series combination of resistors. So we look at the circuit and we start by trying to find the easiest components to combine together. And if we look at that, in the right-most side of the circuit we see that we have a three kilo-ohm in series with a six kilo-ohm resistor. The two on the right-most side of our circuit. So if we combine those two resistors together, they will give us 9 kilo-ohms because they're in series. We then notice that the 9 kilo-ohm resistor, that's a combination of the 3 kilo-ohm and 6 kilo-ohm resistors, is in parallel with the 18 kilo-ohm resistor. So the combination of these resistors, which is 9 K in parallel with 18K, would give us 6K. So we have 6 kilo-ohm resistance as a result of that combination, series and parallel combination of resistors. So we're going to redraw the circuit reflecting those combinations. So RAB looks like this, we have 6K here, we have 6K here, which is our combination of the 9K and 18K, and we have a 10K below that. And so we quickly see that we have a series of three resistors in this case, so RAB is going to be 6K plus 6K plus 10K, equals 22 kilo-ohm resistance. So the combination of the resistors comes out to an equivalent resistance of 22 kilo-ohms.