A current I flowing through the sides of an equilateral triangle of side a. The magnitude of the magnetic field at the centroid of the triangle is

$\frac{9μ_0I}{2πа}$

$B = 3[\frac{\mu_0 I (sin\alpha + sin\beta)}{4\pi r}$]

$\text{here } \alpha = \beta = 60°$

$r = \frac{a}{2\sqrt 3}$

$\Rightarrow B = 3[\frac{\mu_0 I (\sin 60+ \sin 60)}{4\pi r}] = 3[\frac{\mu_0 I \sqrt 3}{4\pi r}] = \frac{9\mu_0 I }{2\pi a}$