4. The state of stress at a point is $\sigma_x = 140 \text{ MPa}$, $\sigma_y = -220 \text{ MPa}$, $\sigma_z = 160 \text{ MPa}$, $\tau_{xy} = -80 \text{ MPa}$, $\tau_{yz} = 100 \text{ MPa}$, $\tau_{xz} = -120 \text{ MPa}$. [Answers: 301.72 MPa, 29.83 MPa, -251.55 MPa, max shear = 276.6 MPa]
a. Calculate the principal stresses.
b. Draw the 3D Mohr's circle for the state of stress.
c. Calculate the maximum shear stress.