The correct answer is 11 ft.
Step-by-step explanation:
The diagonals of a parallelogram bisect each other. This means that the portion marked x+4 is the same length as the portion marked 2x-7; this gives us the equation x+4=2x-7.
Subtract x from each side:
x+4-x=2x-7-x
4=x-7.
Add 7 to both sides:
4+7=x-7+7
11=x