As shown in the figure, a long straight conductor with semicircular...

The formula to calculate the magnetic field $\left({B}_{C}\right)$ produced at the centre of a circular current carrying conductor is given by

${B}_{C} = \dfrac{{\mu}_{o}I}{4\pir}\theta . . . . . . . . . . . . . . . . . . . . . . . ( 1 )$

where, ${\mu}_{0}$ is the permeability of free space, $I$ is the current through the conductor, $r$ is the radius of the circular conductor and $\theta$ is the angle subtended by the arc at the centre.

From the given figure, it is clear that the arc of the conductor makes an angle $\pi$ at its centre.

Substitute the known values of the parameters into equation (1) to calculate the required magnetic field.

$\begin{cases}{B}_{C} & = & \dfrac{4\pi \times {10}^{- 7}N{A}^{- 2} \times 3A}{4 \times \pi \times \dfrac{\pi}{10}m} \times \pi \\ & = & 3 \times {10}^{- 6}T \times \dfrac{{10}^{6}\mu T}{1T} \\ & = & 3\mu T\end{cases}$