b$)$ Determine the maximum design moment $(M_x^{**})$ of a rectangular hollow section $($RHS$)$ of $250**150**9\mathrm{.}0mm$ RHS with Grade C$350,$ shown in Fig. $4.$ The beam is simply supported over a span of $6\mathrm{.}5$ m with full lateral restraint.

$d=250mm$

$b=150mm$

$t=9\mathrm{.}0mm$

$A_9=6600m{m}^2$

$I_x=53\mathrm{.}7m{m}^4$

$Z_x=430**103m{m}^3$

$S_x=533**103m{m}^3$

Fig. $4$ section properties of $\frac{?}{{?}_{50}}**1\mathrm{.}50**9\mathrm{.}0mm$ RHS