We are given that AC is 3 times bigger than AB, therefore we can write an equation AC=AB*3.
We are given that AB=5 centimeters, therefore AC=AB*3=5*3=15. Meaning that AC/AB=3.
A parallel line inside a triangle forms 2 similar triangles. We are given that BE is parallel to CD, therefore the triangles ABE and ACD are similar.
The corresponding side lengths of similar triangles are related by a constant ratio, which is called k.
We found AC/AB=3, therefore k=3.
K is also equal to BE/CD, therefore we can write an equation CD/BE=3.
We are given that CD=27 centimeters, therefore we can write an equation 27/BE=3.