Q2. zz + az + bz +c =0
x2+y2 + (c+id)(x+iy) + (c-id)(x-iy) +k =0
=> x2+y2 + cx - dy + cx + dy + k =0
=> x2+y2 + 2cx + k =0
this represents a circle with centre (-c,0) and radius √c2-k (this may represent a circle or may not)
Q3. the exp is same but as given c2+d2 < k
So the radius turns out to be imaginary,, So this represents nothing i guess.