Answer :

An equilateral triangle has all three sides the same length.

The perimeter of the triangle is the sum of the three sides of the triangle.

Because all three sides are the same, one side is 1/3 * (9A + 12) =

1/3 * 9A + 1/3  * 12 =  3A + 4

The length of each side is 3A + 4.

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