For the given circuit, the power across zener diode is _________ mW.

Total current from the source passing through the series resistor $$R_s = 1\ \text{k}\Omega$$:

$$I_s = \frac{V_{\text{in}} - V_Z}{R_s} \implies I_s = \frac{24 - 10}{1 \times 10^3} = 14 \times 10^{-3}\ \text{A} = 14\ \text{mA}$$

Load current passing through the load resistor $$R_L = 5\ \text{k}\Omega$$:

$$I_L = \frac{V_Z}{R_L} \implies I_L = \frac{10}{5 \times 10^3} = 2 \times 10^{-3}\ \text{A} = 2\ \text{mA}$$

Zener diode current using Kirchhoff's Current Law: $$I_Z = I_s - I_L \implies I_Z = 14 - 2 = 12\text{ mA}$$

Power dissipated across the Zener diode: $$P_Z = I_Z V_Z \implies P_Z = (12\text{ mA}) \times (10\text{ V}) = 120\text{ mW}$$