The number of cans that fit along the lengths of the boxes

The current boxes used to package soda cans have rectangular bases. The 12 cans in a box are stacked in one layer.

could you reply to this with the question so i can ?

ca = ec

cpctc makes it so that corresponding parts of the triangle you proved to be equal are equal, the other options aren't part of the part of the triangles you proved to be equal.