What goes in the box?

Wrote and expression for the difference of p and 4

Will mark brainliest! ( explain the answer too, it can be one sentence)

The perimeter of ? cde is 55 cm. a rhombus dmfn is inscribed in this triangle so that vertices m, f, and n lie on the sides cd , ce , and de respectively. find cd and de if cf=8 cm and ef=12 cm. the answer is not cd=20 and de=15. i have tried that one, it does not work.