To solve this problem it is necessary to take into account the concepts of Intensity as a function of Power and the definition of magnetic field.
The intensity depending on the power is defined as

Where
P = Power
r = Radius
Replacing the values that we have,


The definition of intensity tells us that,

Where,
Magnetic field
Permeability constant
c = Speed velocity
Then replacing with our values we have,

Re-arrange to find the magnetic Field B_0

Therefore the amplitude of the magnetic field of this light is