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$∴a=−3$

It is known that, $a_{n}=ar_{n−1}$

$∴a_{4}=ar_{3}=(−3)r_{3}$ and $a_{2}=ar_{1}=(−3)r$

Now according to the given condition,

$(−3)r_{3}=[(−3)r]_{2}⇒−3r_{3}=9r_{2}⇒r=−3∴a_{7}=ar_{7−1}=ar_{6}=(−3)(−3)_{6}=−(3)_{7}=−2187$

Thus the seventh term of the G.P. is $−2187.$

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