6. Let $A B C D$ be a parallelogram, with three vertices $A, B, C$ having coordinates $(-1,0),(2,1),(0,3)$, respectively. Then the coordinates of vertex $D$ are $\qquad$.
6. $(-3,2)$ Hint: Use the midpoint of $AC$ coinciding with the midpoint of $BD$.

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6. $(-3,2)$ Hint: Use the midpoint of $AC$ coinciding with the midpoint of $BD$.