Omg! fellow k12 student:)
If a pentagon is regular, then all of the sides are the same length. This means that

So, if
, the lengths of the sides evaluate to

So, all the sides of the pentagon are 21 units long, and AB is 21 units long as well, because all sides are the same length.
Answer:
6x-21+5 = -19 Distribute
6x-16 = -19 Combine like terms
6x = -3 Add
x = -1/2 Division
Stop posting on here and do the work yourself I usually don't mind helping people but in some cases, I need to just say do it by yourself look at notes look up the answer there are a million things you can do instead of spam posting all your questions good day