The answer is 38, because if you take away the 15 he bought afterwards (34-15) you get 19, and then since he sold half his collection you would double that (19×2) and that's 38.
The median would stay the same
The standard form of the equation of a circle is:

Where the point (a,b) represents the centre and r is the radius. So, given your information, the equation will be:
Huh? What do you mean I don't understand
2+2=2x+2
-2 to both sides
2=2x
Divide 2
X=1