If the new fraction is 8/12 I will show how to find it.
Let's say the original denominator is x, then the numerator is x-4, so we have (x-4)/x.
Then it says f we add 4 to both, so now we have x/(x+4) this equals 8/12. Now, keep in mind -4 cannot be an answer because it would make x/(x+4) be divided by 0 which wouldn't exist.
A circle equation is : ( x - p )² + ( y - q )² = r², where p and q are coordinates of the center. The position of the source is : ( - 6, - 4 ) and the range : r² = 36 ⇒ r = 6