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In a 30-60-90 right triangle, the side opposite the 30^o angle is equal to one-half the hypotenuse.

Since BC = 12' and BC is the side opposite the 30^o angle, the hypotenuse = 24'.

Also, the side opposite the 60^o angle is the length of the side opposite the 30^o angle times the square root of 3.

So, to find AC:  12 · sqrt(3).