V1=V2, op amp is ideal.
If I know V2 and want to calcuate I, will I get right answer if I pretend there is a wire between V2 and the one below and do mesh analysis on mesh 1. Or I have to do mesh analysis for the whole mesh 2 (which include mesh 1)?

 

Just because V1 = V2 does not mean that V2 is equal to the voltage on the bottom wire (I assume that is the node you are declaring to be ground).

Remember that a virtual short does NOT mean that the voltage is 0 V, just that the voltage on the two nodes are the same, possibly non-zero, value.

For this problem you don't need to do any sophisticated analysis at all.

What is Vout in terms of V3? What is V3 in terms of V2? What is V2 in terms of V1? What is V1 in terms of Vin and Vout?

 

An ideal opamp is a dependent voltage source, here hooked up as a unity gain buffer, so it can just be modeled as a voltage source with value = V1. You could make a mesh that includes V1, but it would reduce to a voltage source with value V1, driving R2.

Are you looking for a DC current, a transient response to a step, impulse, or sine wave, or an expression for current as a function of frequency?

Regards