How to calculate this two transistors Constant Current Source circuit?
With R2, Q2 and R3 you set the base of Q2 and Q1 to the level of UR3, let's call this the regulation path conduction IReg. Current flowing in this path is 

\$IReg = (E1-0.7V)/(R2+R3) = ~29.97mA\$

By choosing the ratio between R1 and R2 you define the ratio between the current through Q1 and Q2 - as the two bases are fixed to the same voltage level. When you try to draw more current on IOut, the increasing voltage drop on R1 will cause a lower collector-base voltage drop on Q1 and therefore regulate the current to 

\$IOut = IReg*R1/R2 = 299.7mA\$

As correctly stated in the comment, if the calculation has to be accurate one has to take the base current of Q1 into account that will additionally flow through R3.