You can put this solution on YOUR website! If by "height" is meant "altitude", then this being in the plane of the deck. cuts this equilateral triangle shape into two congruent special 30-60-90 right triangles.
Do you see? Pythagorean Theorem, and hypotenuse is the length of one side of the deck.
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Half the equilateral triangle, special 30-60-90
leg, 27 feet
leg, x
hypotenuse, 2x
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side of the equilateral triangle,