How can I solve this clipper circuit?
I'll give you a general answer:

Usually, when encountering a circuit with 1 or more ideal diodes (as the question in Albanian is stating) you will split the behaviour of your components in different cases. The cases are selected according whether the diodes are in conductive or non-conductive polarity. This will yield 4 cases when you have 2 diodes. Usually one of those cases can be omitted from the start because it will be contradictory leaving three segments for the U-I-Diagram of the whole circuit.

You then can draw replacement circuits where you replace the diodes by shorts or open ends.

Alternatively there's the possibility to generate the U-I-diagram graphically by „adding“ the curves of all parts on either the I or U axis depending on parallel or series connection. 

Having gained the complete U-I diagram it is easy to calculate the output behaviour.