HW$2\mathrm{.}3.$ Analysis of an Extruded Triangular Thin$-$Walled Tube

A hollow tube extruded with the shape of an equilateral triangle is shown in the figure below. Its base is

$3\mathrm{.}5$ in $.$ long and its wall thickness is $0\mathrm{.}15$ in $.$ The tube is subjected to a torque of $2000in*l{b}_f.$ If the tube is

fabricated from aluminium and is $42$ in $.$ long, determine the maximum value of the average shearing stress

and the angle of twist of the tube.