Answer:
x = 8.699 10⁻³ m
Explanation:
The proton feels an electric charge that is the opposite direction of speed, let's look for acceleration using Newton's second law
F = m a
F = q E
a = q E / m
a = 1.6 10⁻¹⁹ 1500 / 1.67 10⁻²⁷
a = 1,437 10¹¹ m / s²
Now we can use kinematic relationships
v² = v₀² - 2 a x
When at rest the speed is zero (v = 0)
x = v₀² / 2 a
Let's calculate
x = 50,000² / (2 1,437 10¹¹)
x = 8.699 10⁻³ m