Draw the free body diagram of the beam as follow:
Apply force equilibrium condition along axial direction.
Apply force equilibrium condition along vertical direction.
Apply moment equilibrium condition about node A.
Apply the equation for moment of inertia,

, for a hollow rectangle.
Here, the width of the outer rectangle is

, the height of the outer rectangle is

, the width of the inner rectangle is

, and the height of the inner rectangle is

,
Substitute 100 mm for

, 150 mm for

, 90 mm for

, and 140 mm for

.
Apply the equation for area,

, of the rectangle.
Substitute 100 mm for

, 150 mm for

, 90 mm for

, and 140 mm for

.
Apply the formula for static moment of area,

, at point

.
Substitute

for

,

for

,

for

, and

for

.
Apply the formula for shear stress at point A.
Substitute

for

,

for

,

for

, 90 mm for

, and 100 mm for

.
Apply the formula for the normal stress at point A.
Apply the formula for the average normal stress at point A.
Substitute

for

.
Apply the formula for the radius of Mohr’s Circle for point A.
Apply the formula for the principle stresses at point A.

.
Substitute

for

and

for

to calculate the maximum and the minimum principal stresses at point

.
Therefore, the stresses at point

are

and

.
Apply the formula stresses at point B by using Equation 9.10.
Substitute

for

,

for

,

for

,

for

and

for

.
Point

lines on the bottom surface of the boom, so there is no shear stress.
Thus, the stresses at point

are

and

.