Physics- Magnetic Moment/Dipole?

Question

A student makes a short electromagnet by winding 300 turns of wire around a wooden cylinder of diameter 5 cm. The coil is connected to a battery producing a current of 4 A in the wire.

a. What is the magnetic moment of the device?

b. At what axial distance will the magnetic field of this dipole have the magnitude of 5 micro-T?

Answer 1

a. A = πr² = π x (0.025 m)² = 1.96 x 10^-3 m²

µ = NIA = 300 turns x 4 A x (1.96 x 10^-3 m²) = 2.35 Am²

b. B = µ₀NIr² / 2(x² + r²)^(3/2)

That's the only equation I know of. Use that to solve for x, the distance.