Answer:
The answer is 6 feet.
Explanation:
Let's take a right angled triangle ABC which has 3 sides of lengths a, b and c. c is the longest side or the hypotenuse.
Then by Pythagoras rule:
Let's take the length of the unknown leg in the figure as
.
Now lets substitute the values from the figure to above equation.

=

=

=

=
or

Length of a triangle cannot be a negative value, so we can take +6 as the value of x.
So the answer is 6 feet.