Answer
2.0 T
Work Step by Step
Use equation 32–2 to relate the cyclotron frequency and the magnetic field strength.
$$f=\frac{qB}{2\pi m}$$
$$B=\frac{2\pi mf}{q}$$
$$=\frac{2\pi (1.67\times10^{-27}kg)(3.1\times10^7Hz)}{1.60\times10^{-19}C} $$
$$=2.0T$$