figure on scale 
generally it is well known  that angle inscribed  by  diameter is $90$,but because on GRE figures are   not  drawn on scale,that why i want to make sure that  my assumption about $y$ is $90$ ,generally  in some problem,it is always possible to redraw   figures so that create different  angles and different  form,that why i want to be sure that if such kind of figure would be on GRE, i would not make any mistake,,thanks in advance

The answer is indeed, $y = 90^\circ$, since you are given that the angle is inscribed by the diameter. And you note, this is a fairly well know theorem. That is, by Thales Theorem, we know that $y = 90^\circ$.
So in this case, you can rest assured you are given all the information you need, drawn to scale or not. 
