$r = dfrac{m_ev}{|e|B}$

where $m_e = 9.11 times 10^{-31}:kg$ is mass of the electron and $|e| = 1.6 times 10^{-19}:C$ is the absolute value of the charge of the electron.

Magnitude of the magnetic field is given as $B =0.45:T$ and the speed of the electron is given as $v = 6.27 times 10^5:m/s$

So the radius of the electron’s path is

$r = dfrac{9.11 times 10^{-31} times 6.27 times 10^5}{1.6 times 10^{-19} times 0.45} = dfrac{9.11 times 6.27}{1.6 times 0.45} times 10^{-31} times 10^5 times 10^{19}$

$$
r =7933times 10^{-9}:m = 7933:nm
$$

$B = dfrac{m_e v}{|e| r}$

where $m_e = 9.11 times 10^{-31}:kg$ is the mass of the electron and $|e| = 1.6 times 10^{-19}:C$ is the absolute value of electron’s charge.

Radius of the circular path is $r=8.5:mm = 8.5 times 10^{-3}:m$ and the speed of the electron is $v=5.1 times 10^6:m/s$. So the required magnetic field is

$B = dfrac{9.11 times 10^{-31} times 5.1 times 10^6}{1.6 times 10^{-19} times 8.5 times 10^{-3}} = 3.42 times 10^{-3}:T$