Power is related to voltage and current by the relation P = V*I = V^2/R. In the circuit, two resistors R1 and R2 are connected in parallel across the voltage source of 10 V. A current of 10 mA flows though the resistor R2. This gives the resistance of R2 as 10/10*10^-3 = 1 k ohm.
The equivalent resistance of R1 and R2 is given by R where 1/R = 1/R1 + 1/R2
=> 1/R = 1/5000 + 1/1000
=> R = 2500/3
The power through the circuit is 10^2/(2500/3) = 0.12 W