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put this solution on YOUR website!i'm guessing you have geometry. it's pretty simple. basically, there is theorem that states something about diagonals of parallelograms. i remember that 1a times 2a = 1b times 2b. so if 1a=3, 2a must equal to 3 because there are equadistant from each diagonal.
------------a_________b
------------/ /
-----------/ /
----------/________/
----------c d
if there is a line from A to D and C to B, then there would be a point where two line meets. but when each line cuts in half it bisects. i hope you know what that means. so if a to the middle point is 3, then the middle point to d is 3 as well. i hope that helps for you...