If the vector $\overline{\mathrm{a}}=2\overline{\mathrm{i}}+3\overline{\mathrm{j}}+6\overline{\mathrm{k}}$ and $\overline{\mathrm{b}}$ are collinear and $\overline{\left|\mathrm{b}\right|} =21$ then $\overline{\mathrm{b}}=$
 

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