Answer:
18
Step-by-step explanation:
Since you know the perimeters are equal, write your equation.
x+6 + x+6 + x+6 = x + x+6 + 2x-6
combine like terms
3x + 18 = 4x
isolate your x by subtracting 3x from each side
18 = x
check your answer:
3x + 18 = 72
4x = 72
