Hi Saeed,
For this type of circuit you can't simplify using only equivalent resistors. You need to use Kirchoff's Circuit Laws (Voltage & Current) to solve this circuit.
1) Label each branch with a current name (I1, I2, I3, etc)
2) Use Kirchoff's Current Law to find an equation for each Node
3) Use Kirchoff's Voltage Law to find an equation for each Loop
4) Use the math you learned in class to solve the system of équations (isolate variables and substitute).
Here's a great link on applying Kirchoff's circuit laws:
http://www.electronics-tutorials.ws/dccircuits/dcp_4.htmlBut Kevin is right, you need to solve this yourself!