Local Stability

$4\mathrm{.}4-1$ An HSS $108\frac{3}{16}$ is used as a compression member with one end pinned

and the other end fixed against rotation but free to translate. The length

is $12$ feet. Compute the nominal compressive strength for A$500$ Grade B

steel $(F_y=46$ksi$).$ Note that this is a slender$-$element compression member,

and the equations of AISC Section E$7$ must be used. $($Note: I know the answers is $161$ kips from the back of the book, I$\text{'}$m just confused how to get there$)$