We remember that the escape velocity formula is given by:
[tex]v=\sqrt{\frac{2Gm}{r}}[/tex]
Now, from this we determine the radius as follows:
[tex]v^2=\frac{2Gm}{r}\Rightarrow r=\frac{2Gm}{v^2}[/tex]
Then:
[tex]r=\frac{2(6.67\ast10^{-11})(1.56\ast10^{15})}{(5)^2}\Rightarrow r=8324.16m[/tex]
So, the radius of "planet" Apgar is approximately 8324.16 meters.
