In the question, he told me from the begging that it is an ideal op amp .. so A(cm) =0 !! How can I find the CMRR then !!
I tried to analyze the circuit to be sure!! And i got the same !
Hint - it may be an ideal op-amp but the overall circuit is not ideally suited to reject common mode voltages applied to both input nodes simultaneously. You have to work out what change in output voltage results from a change in input voltage. When you do part (b) you will then see that the overall circuit is no longer ideal.
A(cm) =0 !! I tried to analyze the circuit to be sure!! And i got the same !
Then you did it incorrectly but that image you posted is too small for my old eyes to read.