Figure shows a straight wire of length $l$ current $i$. The magnitude of magnetic field produced by the current at point $P$ is

The given situation can be redrawn as follow.

As we know the general formula for finding the magnetic field due to a finite length wire

$B=\frac{{\mu }_0}{4\pi }\cdot \frac{i}{r}\left(\sin {\varphi }_1+\sin {\varphi }_2\right)$

$\text{ Here }{\varphi }_1=0^{\circ },\varphi ={45}^{\circ }$

$\therefore B=\frac{{\mu }_0}{4\pi }\cdot \frac{i}{r}\left(\sin 0^{\circ }+\sin {45}^{\circ }\right)=\frac{{\mu }_0}{4\pi }\cdot \frac{i}{\sqrt{2}l}\Rightarrow B=\frac{\sqrt{2}{\mu }_{0}i}{8\pi l}$