Answer:
(1) Each side of triangle A is changed by a factor of 1/2.
(2) The unknown side length in triangle B has a measure of 7.5 units.
Explanation:
It is given that Alejandro reduced triangle A proportionally.
It means triangle A and B are similar and their corresponding sides are proportional.

From the given figure it is clear that the side of length 12 units is reduce to 6 units.


Each side of triangle A is changed by a factor of 1/2.
Let the unknown side of triangle B be x.

On cross multiplication we get

Divide both sides by 2.

Therefore, the unknown side length in triangle B has a measure of 7.5 units.