The distance from corner to $P$ is,

$r=a \sqrt{a}$

The magnetic field at centre $P$ is,

$B=\frac{\mu_{0} i}{\pi a} r$

$=\frac{\mu_{0} i}{\pi a} a \sqrt{a}$

$=\frac{4 \times 10^{-7} \times 3}{\pi \times 0.2}(0.2 \sqrt{0.2})$

$=120 \sqrt{2} \times 10^{-7}$

$=12 \sqrt{2} \times 10^{-6} T$