It's basically mandatory to memorize the three most common special triangles: the 345 triangle, the 306090 triangle, and the right isosceles. They appear constantly. Sometimes they appear in my sleep. Anyway, we recognize that the upper triangle in the given rectangle is a 345 triangle that's been expanded by a factor of 2 to get a 6810 triangle. (We know it's a right triangle because a rectangle must have right angles at its corners.) The missing side on the top of the rectangle must therefore be 8. Since we know that halfway around the rectangle is 6 + 8, giving us 14, the full perimeter must be 14x2, or 28, giving us answer choice C.
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