The measure of angle B is 120 degrees
What is rigid transformation
A rigid transformation is a type of transformation in geometry that preserves the size and shape of a figure.
When a rigid transformation is applied to a geometric object, the object's size and angles remain unchanged, and only its position and orientation in the plane are changed.
There are three main types of rigid transformations:
Reflection: This involves flipping the object across a line (mirror line). This is a type of rigid transformation hence the angles and size of the object is preserved
With this information we can say that angle B is 120 degrees