Answer:
x=7
Explanation:
4 (3x+4)= 100 ft (Perimeter = sum of all 4 sides)
12x + 16 =100 ft (Multiply length of one side by 4, which is number of side on a square)
12x = 84 (This is what is left after subtracting 16 form both sides of the equation making the equation still true)
x = 7 (84 divided by 7 to let X stand alone and get the value of x)