Answer
$B = 8.0\times 10^{-4}~T$
Work Step by Step
We can write the expression for the magnetic field produced by a current in a straight wire:
$B = \frac{\mu_0~i}{2~\pi~R}$
To find the magnitude of the magnetic field at point $b$, we can double the magnetic field due to one of the current-carrying wires:
$B = 2\times \frac{\mu_0~i}{2~\pi~R}$
$B = \frac{\mu_0~i}{\pi~R}$
$B = \frac{(4\pi\times 10^{-7}~H/m)~(10~A)}{(\pi)~(5.0\times 10^{-3}~m)}$
$B = 8.0\times 10^{-4}~T$
