What is the cross section formed by a plane that intersects three faces of a cube

Find the length of cu, the triangles are similar.

Given the two similar triangles, how do i find the missing length? if a=4, b=5, and b'=7.5 find a'

Rhombus adef is inscribed into a triangle abc so that they share angle a and the vertex e lies on the side bc . what is the length of the side of the rhombus if ab=c, and ac=b.