Acceleration = (0-9) / 3 = -3m/s^2
Average acceleration over a time interval lasting
is

where
is the difference in the jet's final and initial velocities. It's coming to a rest, so

so the average acceleration has magnitude 8.9 m/s^2 and is pointing West (the direction opposite the jet's movement, which should make sense because the jet is slowing down).
Explanation:
Formula for angle subtended at the center of the circular arc is as follows.

where, S = length of the rod
r = radius
Putting the given values into the above formula as follows.

= 
= 
= 
Now, we will calculate the charge density as follows.

= 
= 
Now, at the center of arc we will calculate the electric field as follows.
E = 
= 
= 34.08 N/C
Thus, we can conclude that the magnitude of the electric eld at the center of curvature of the arc is 34.08 N/C.
The element would have 52 protons because atomic number gives protons.