Find the principal stresses and the orientation of the principal axes of stresses for the following cases of plane stress.

a. \(\sigma_{x x}=40 \mathrm{MPa}, \sigma_{y y}=0 \mathrm{MPa}, \tau_{x y}=80 \mathrm{MPa}\)

b. \(\sigma_{x x}=140 \mathrm{MPa}, \sigma_{y y}=20 \mathrm{MPa}, \tau_{x y}=-60 \mathrm{MPa}\)

c. \(\sigma_{x x}=-120 \mathrm{MPa}, \sigma_{y y}=50 \mathrm{MPa}, \tau_{x y}=100 \mathrm{MPa}\)