Answer:
33.6cm
Explanation:
I'm assuming the length is 3x as there are quite a few typos
perimeter = 2 x length + 2 x width
84 = 2 x 3x + 2 x 12x
84 = 6x + 24x
84 = 30x
84/30 = 2.8
x = 2.8
12x = 2.8 x 12 = 33.6 cm
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