Answer
$B = 1.2\times 10^{-4}~T$
Work Step by Step
The equivalent resistance of the $20~\Omega$ resistor and the $30~\Omega$ resistor in series is $50~\Omega$
Since the potential difference across these resistors is $60~V$, the current in this section of the circuit is $I = \frac{60~V}{50~\Omega} = 1.2~A$
We can find the magnetic field strength at point A:
$B = \frac{\mu_0~I}{2~\pi~r}$
$B = \frac{(4\pi\times 10^{-7}~T~m/A)~(1.2~A)}{(2~\pi)~(2.0\times 10^{-3}~m)}$
$B = 1.2\times 10^{-4}~T$