Solve for a_n
a_{n}=-2n^{2}-12
Solve for n
n=\frac{\sqrt{-2a_{n}-24}}{2}
n=-\frac{\sqrt{-2a_{n}-24}}{2}\text{, }a_{n}\leq -12
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a_{n}=-2n^{2}-12
Subtract 7 from -5 to get -12.
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