$M = 600\hspace{0.05cm}Nm = 600\hspace{0.05cm}\times\hspace{0.05cm}10^3\hspace{0.05cm}Nmm\\ y = \frac{150}{2} = 75\hspace{0.05cm}mm\\ I = \frac{bd^3}{12} = \frac{50\hspace{0.05cm}\times\hspace{0.05cm}150^3}{12} = 14.06\hspace{0.05cm}\times\hspace{0.05cm}10^6\hspace{0.05cm}mm^4$

$\frac{M}{I} = \frac{\sigma_b}{y}$ $\frac{600\hspace{0.05cm}\times\hspace{0.05cm}10^3}{14.06\hspace{0.05cm}\times\hspace{0.05cm}10^6} = \frac{\sigma_b}{75}$

$\sigma_b = 3.20\hspace{0.05cm}N/mm^2$