The magnetic field at $P$ is
$B=\frac{\mu_{0} n I}{2}(\cos \theta+\cos \theta)$
where $n$ is number of turns, $I$ the current.
Given, $n=100, I=5 A$ and $\theta=45^{\circ}$
$\therefore B=\frac{\mu_{0} \times 100 \times 5}{2} \times \frac{2}{\sqrt{2}}$
$\Rightarrow B =250 \sqrt{2} \,\mu_{0}$
