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3a - 2 + 5a + 3 + 6a + 1 = 86
14a + 2 = 86
14a = 86 - 2
14a = 84
a = 84/14
a = 6
lengths :
3a - 2 = 3 × 6 - 2 = 18 - 2 = 16 cm
5a + 3 = 5 × 6 + 3 = 30 + 3 = 33 cm
6a + 1 = 6 × 6 + 1 = 36 + 1 = 37 cm
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