Apply the junction rule at point a in Figure 21.55.
$I_3 = I_1 + I_2$
This is College Physics Answers with Shaun Dychko. The Junction Rule says that all the currents going into a point equals the sum of all the currents leaving that point or that junction. And so this junction here has a current I3 going into it. So we have the total of all the currents going in of which there’s only one, so it’s I3 equals the total of the currents coming out which is I1 and I2. So there’s our Junction Rule written out separately.