Answer is 5

Voltage across RS = 22 – 15 = 7V
Current through RS = I = \(\frac 7{35}\) = \(\frac 15\) A
Current through 90Ω = I2 = \(\frac{15}{90}=\frac 16\) A
Current through zener = \(\frac 15-\frac 16=\frac 1{30}\) A
Power through zener diode
P = VI
