The length of the metal frame that surrounds the trampoline is the difference in circumference between the frame and the mat.
Circumference of the mat = π * diameter = 3.14 * 16 = 50.24 feet
Circumference of the frame = π * diameter = 3.14 * 18 = 56.52 feet
Length of the metal frame = Circumference of the frame - Circumference of the mat
Length of the metal frame = 56.52 - 50.24 = 6.28 feet
Rounded to the nearest tenth, the length of the metal frame is 6.3 feet.