The magnitude of the magnetic field must be 29.0 nT
This is found using the equation
B=uNI/cosQ
Where B is the magnetic field strength, u is the magnetic permeability of free space, N is the unit normal vector pointing out of the surface, and Q is the angle between the unit normal vector and the magnetic field.
In this case the angle between the unit normal vector and the magnetic field is 90 degrees - 28.5 degrees = 61.5 degrees