For steel:
$\sigma_{st} \, A_{st} = P_{st}$
$\sigma_{st} (0.5) = 12$
$\sigma_{st} = 24 \, \text{ ksi}$ answer
For aluminum:
$\sigma_{al} \, A_{al} = P_{al}$
$\sigma_{al} \, (0.5) = 12$
$\sigma_{al} = 24 \, \text{ ksi}$ answer
For bronze:
$\sigma_{br} \, A_{br} = P_{br}$
$\sigma_{br} (0.5) = 9$
$\sigma_{br} = 18 \, \text{ ksi}$ answer