The value of power dissipated across the Zener diode is

Voltage across the load resistance RL is 15 V so current across it is$\frac{15}{90}=\frac{1}{6}\mathrm{A}$ and voltage across 35 ohm resistance is (22-15)= 7 volt
So current in 35 ohm resistance is $\frac{7}{35}=\frac{1}{5}\mathrm{A}$ then current through Zener is $\left(\frac{1}{5}-\frac{1}{6}\right)-\frac{1}{30}\mathrm{A}$
Power across Zener is $\mathrm{V} \mathrm{x} 1=15\mathrm{x}\frac{1}{30}=0.5 \mathrm{W}$