A single crystal of aluminum is oriented for a tensile

A single crystal of aluminum is oriented for a tensile test such that its slip plane normal makes an angle of 28.1( with the tensile axis. Three possible slip directions make angles of 62.4(, 72.0(, and 81.1( with the same tensile axis.
(a) Which of these three slip directions is most favored?
(b) If plastic deformation begins at a tensile stress of 1.95MPa (280psi), determine the critical resolved shear stress for aluminum.