hehehehe.... I am sure your tutor is not going to deduct marks for saying that instead of x^2+y^2=2^2,.. that answer actually shows that you know what x^2+y^2=2^2 really means.

Was there a reason for solving for y?
[itex]y= \pm \sqrt{4- x^2}[/itex] says nothing that [itex]x^2+ y^2= 4[/itex] doesn't and I would consider the second form simpler.

Relevant to your actual question, the problem did NOT say "find the equation"- it said "Identify the curve by finding a Cartesian equation for the curve."