check the picture below.
as you can see on the example on the left-side there, two tangent lines to a circle, by the time they intersect each other, they have the same length, x = x, z = z and y = y, because they're tangent to that circle and are intersecting.
therefore, the missing part in this circle is 8, as you see there in the right-side.
so the perimeter is 24 + 22 + 18.
