I found your complete question in another source.
See attached image.
For this case what you should see is that you have two similar triangles.
We then have the following relationship:
((AB) / (BX)) = ((CD) / (DX))
We cleared CD:
CD = ((AB) / (BX)) * (DX)
Substituting the values:
CD = ((12) / (5)) * (2.5)
CD = 6
Answer:
CD = 6
See attached image.
For this case what you should see is that you have two similar triangles.
We then have the following relationship:
((AB) / (BX)) = ((CD) / (DX))
We cleared CD:
CD = ((AB) / (BX)) * (DX)
Substituting the values:
CD = ((12) / (5)) * (2.5)
CD = 6
Answer:
CD = 6