Answer:
29 feet
Imagine you are on an open plane.
you walk 11 feet forward and your friend 10 feet. you are now 21 feet apart. now put imaginary walls that stopped both of you at that 21 feet.
now, you go 12 feet to your right, and your friend 8 feet to their right. imagine you are pushing a wall in front of you.
at that point the side walls are 20 feet apart.
you and your friend are at opposite corners of a 20 by 21 foot room.
at that point it's just a matter if applying the Pythagorium theory.