You need more information to answer this question. Is there a table or percentage for that age group?
(2x - 6)^2 = (2x - 6)(2x - 6) = 4x^2 - 12x - 12x + 36 = 4x^2 - 24x + 36
The polygon would have 18 sides. Hope this helps!
Answer:definition of midpoint, angle BCA is congruent to angle DCA