If $\overline{\mathrm{a}},\overline{\mathrm{b}}$ and $\overline{\mathrm{c}}$ are non-zero vectors such that $\overline{\mathrm{a}}$ and $\overline{\mathrm{b}}$ are not perpendicular to each other, then the vector $\overline{\mathrm{r}}$ which is perpendicular to $\overline{\mathrm{a}}$ and satisfying $\overline{\mathrm{r}}\times \overline{\mathrm{b}}=\overline{\mathrm{c}}\times \overline{\mathrm{b}}$ is