If a pentagon is regular, then all of the sides are the same length. This means that
[tex] 5x-4=2x+11 \iff 5x-2x = 11+4 \iff 3x=15 \iff x = 5 [/tex]
So, if [tex] x=5[/tex], the lengths of the sides evaluate to
[tex] 5x-4 = 25-4=21,\quad 2x+11=10+11=21 [/tex]
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.
