Hello! Hope this is helpful. I'll give you an example so you can figure this out on your own.
If a line segment is AD + DE, and AD is (x + 4) and DE is (x + 8), then AE is (x + 4) + (x + 8).
Combine like terms, terms with the same variable (x) or none.
x + 8 + x + 4 = 2x + 12.
The whole line is the length 2x + 12.
With this, try solving it!