Answer
The turning point is $~~r = 2.5~R_s$
Work Step by Step
The turning point occurs when the kinetic energy is zero.
We can use conservation of energy to find $U$ when $K = 0$:
$K+U = -2.0\times 10^9~J$
$0+U = -2.0\times 10^9~J$
$U = -2.0\times 10^9~J$
On the graph, we can see that $~~U = -2.0\times 10^9~J~~$ at $~~r = 2.5~R_s$
Therefore, the turning point is $~~r = 2.5~R_s$
