<span> We're given that x=25 when t=2: </span>
<span>25 = 3 + 12(2) + (1/2)a(2)^2 </span>
<span>Thus a = -1 cm/sec^2</span>
I'm not sure, I think it's option A.
Let me know if I'm wrong!
Explanation:
It is given that,
Mass of lithium, 
It is accelerated through a potential difference, V = 224 V
Uniform magnetic field, B = 0.724 T
Applying the conservation of energy as :


q is the charge on an electron

v = 78608.58 m/s

To find the radius of the ion's path in the magnetic field. The centripetal force is balanced by the magnetic force as :



r = 0.0078 meters
So, the radius of the path of the ion is 0.0078 meters. Hence, this is the required solution.