Terms in arithmetic progression : $$(2x - 4) , (4x + p) , (6x - 12)$$
=> Difference between first two terms is equal to the difference between last two terms
=> $$(4x + p) - (2x - 4) = (6x - 12) - (4x + p)$$
=> $$2x + p + 4 = 2x - 12 - p$$
=> $$2p = -12 - 4 = -16$$
=> $$p = \frac{-16}{2} = -8$$
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