$7-10*$ Two L$10210212\mathrm{.}7-mm$ structural steel angles

$($see Appendix B for dimensions$)$ are attached back to back

to form a T$-$section, as shown in Fig. P$7-10.$ Determine the

maximum resisting moment $M_r$ that can be supported by

the beam if the maximum flexural stress must be limited to

$120$MPa.

Figure P$7-10$