Solving a simple Ohms law circuit (parallel and series circuit)

I know there is a serial and parallel circuit

I know R1 = 2k, R3 = 1k and i know Vout which is 1V. And i know 5V is coming from supply. I need to calculate the size of resistor R2.

Can somebody explain me with exact equations to calculate R2, and currents which run trough resistors (step by step please)

I can calculate :
1) IR1 = 5-1V/2000ohms = 2mA
2) UR1 = 0,002A*2000ohms=4V voltage drop on R1

How do i go on finding R2?

Help will be much appreciated!

Assuming there's no current drawn off the Vout connection...
You know there's 2mA flowing through R1
R3 has 1/1000 = 1mA
So 2-1=1mA through R2 - so it must be 1k also

or alternatively - R2&R3 in parallel have one fifth of 5V across them - so they have a fifth of the overall resistance.
2000 ohms is four of the fifths so 500 ohms make the other fifth up - so R2 & R3 in paralall must equal 500 ohms. Which is what you get from 2 x 1000 ohms in parallel