Chapter 8 - Section 8.2 - Inconsistent and Dependent Systems and Their Applications - Exercise Set - Page 903: 29

Equation for I2 to keep the traffic moving is $ x+6=z+12$.

Here the number of cars entering I2 is $ z+12$ and the number of cars leaving I2 is $ x+6$, as per the figure. And to keep the traffic going, the number of cars entering should be equal to the number of cars leaving. So, $ x+6=z+12$