Answer:
The plate's surface charge density is 
Explanation:
Given that,
Speed = 9800 km/s
Distance d= 75 cm
Distance d' =15 cm
Suppose we determine the plate's surface charge density?
We need to calculate the surface charge density
Using work energy theorem


Here, final velocity is zero
...(I)
We know that,


...(II)
From equation (I) and (II)

Charge is negative for electron

Put the value into the formula


Hence, The plate's surface charge density is 