You do what is asked for in the parentheses then multiply by the number outside of them.
Answer:CD = x = 4 units
Explanation:We are given that the two triangles are similar. This means that we can set the similarity proportionality as follows:

W e have:
BC = 20 - x
AC = 20
CD = x
CE = 5
Substitute in the above proportionality and solve for x as follows:

4x = 20 - x
5x = 20
x = 4
Based on he above:
CD = 4 units
BC = 20 - x = 20 - 4 = 16 units
Hope this helps :)
Any second part or anything else to look at?