Calculating the minimum and maximum \$R_{load}\$ for a zener voltage regulator
The maximum load resistance of (say) infinity will push 10.5 mA into the zener. This is because 6.8 volts will be across Rs and, as Rs is 650 ohms, there will be a current into the unloaded zener of 10.46 mA.

The minimum load resistance is that value of load resistance that forms a potential divider with Rs to produce 8.2 volts at the output with the zener removed so, RL will be 784 ohms because 784/(784 + 650)*15 = 8.2 volts.