Answer:
a)

b)

c)

d)

Step-by-step explanation:
a) We can find the electron's speed by knowing the kinetic energy:

Where:
K: is the kinetic energy = 1.59 keV
m: is the electron's mass = 9.11x10⁻³¹ kg
v: is the speed =?

b) The electron's speed can be found by using Lorentz's equation:
(1)
Where:
F: is the magnetic force
q: is the electron's charge = 1.6x10⁻¹⁹ C
θ: is the angle between the speed of the electron and the magnetic field = 90°
The magnetic force is also equal to:
(2)
By equating equation (2) with (1) and by solving for B, we have:

c) The circling frequency is:

Where:
T: is the period = 2π/ω
ω: is the angular speed = v/r

d) The period of the motion is:

I hope it helps you!