Answer
$2.7\times10^{-2}T$ upward
Work Step by Step
See Example 20-6. The magnetic force is the centripetal force.
$$qvB=m\frac{v^2}{r}$$
$$B=\frac{mv}{qr}=\frac{p}{qr}$$
$$=\frac{4.8\times10^{-21}kg \cdot m/s}{(1.60\times10^{-19}C)(1.1m)}$$
$$=2.7\times10^{-2}T$$
Assume the protons are moving in a horizontal circle, orbiting clockwise as seen from above. The magnetic field points upward. This causes an inward-pointing (centripetal) force which makes the positively-charged protons orbit clockwise.
