Answer
The proton reaches a turning point at $~~x = 1.5~cm$
Work Step by Step
We can find the total energy at $x = 3.5~cm$:
$E = K+U$
$E = K+q~V$
$E = (4.00~eV)+(1e)(3.00~V)$
$E = 7.00~eV$
By conservation of energy, the proton can reach a location where the electric potential is $7.00~V$
Therefore, the proton reaches a turning point at $~~x = 1.5~cm$
