# Calculating current to produce magnetic field of Earth

Physics Help!
Suppose that the magnetic field of the Earth were due to a single current moving in a circle of radius 2988 km through the earth’s molten core. The strength of the Earth’s magnetic field on the surface near a magnetic pole is about 6.00E-5 T. About how large a current would be required to produce such a field?

I have to use the equation B=((mu_naught*i)/2)* (x^2/(x^2+R^2)^(3/2))
and then for i get:
i=(2*(x^2+R^2)^(3/2))/(mu_naught*R^2)

How do I get x?

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