Complete the construction of triangle ABC.

• Draw a line segment AB of length $5 \text{ cm}$.
• At point A, construct an angle of $120^\circ$. This is done by drawing an arc centered at A, then marking $60^\circ$ and $120^\circ$ along this arc. Draw a ray from A through the $120^\circ$ mark.
• With point B as the center, open the compass to a radius of $10 \text{ cm}$. Draw an arc that intersects the ray from A. Label the intersection point C.
• Draw a line segment from B to C to complete the triangle ABC.

Using a protractor, measure the angle at vertex B, which is $\angle ABC$.
From the provided figure, the measured angle is $34.6^\circ$.

To verify this measurement using trigonometry:
Given $AB = 5 \text{ cm}$, $BC = 10 \text{ cm}$, and $\angle BAC = 120^\circ$.
Using the Sine Rule:

The sum of angles in a triangle is $180^\circ$:

The measured value of $34.6^\circ$ is consistent with the calculated value, allowing for typical construction and measurement tolerances.