Answer
$(0,-3)$.
Work Step by Step
We know that $a(7,-1)+b(-9,-2)=(27,6)$. Thus $7a-9b=27$ and $-a-2b=6\\7a+14b=-42$.
Thus $7a+14b-(7a-9b)=-42-27\\23b=-69\\b=-3$.
Thus $7a-9(-3)=27\\7a=0\\a=0$
Thus the vector is $(0,-3)$.
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