#### Answer

The horizontal distance between point A and point B is 18 feet.

#### Work Step by Step

Let $a = 10~ft$, let $b = 10~ft$, and let angle $C = 128^{\circ}$.
We can use the law of cosines to find $c$, the length of the line opposite the angle $C$:
$c^2 = a^2+b^2-2ab~cos~C$
$c = \sqrt{a^2+b^2-2ab~cos~C}$
$c = \sqrt{(10~ft)^2+(10~ft)^2-(2)(10~ft)(10~ft)~cos~128^{\circ}}$
$c = \sqrt{323.13~ft^2}$
$c = 18~ft$
The horizontal distance between point A and point B is 18 feet.