Chapter 8 - Section 8.2 - Inconsistent and Dependent Systems and Their Applications - Exercise Set - Page 903: 30

Equation for I3 to keep the traffic moving is $ z+8=y+6$.

Here, the number of cars entering I3 is $ y+6$ and the number of cars leaving I2 is $ z+8$, 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, $ y+6=z+8$