Answer
The particle of mass $m$ could approach the particle of mass $M$ until reaching the closest point $r$ where $U = E$
The particle of mass $m$ will then escape to the right since $E \gt U$.
Work Step by Step
The particle of mass $m$ could approach the particle of mass $M$ until reaching the closest point $r$ where $U = E$
Then, since the slope of the $U$ versus $r$ graph is negative at this point, there will be a repelling force between the particles.
The particle of mass $m$ will move to the right, farther away from the particle of mass $M$. The particle of mass $m$ will then escape to the right since $E \gt U$.
