3. When $k=$ $\qquad$, the principal value of the argument of the complex number $\left(2 k^{2}-3 k-2\right)+$ $\left(k^{2}+k-6\right)$ i is $270^{\circ}$.
$3 . k=-\frac{1}{2}$.

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$3 . k=-\frac{1}{2}$.