The unit vector parallel to the resultant of the vectors $\overrightarrow{A}=4\hat{i}+3\hat{j}+6\hat{k}\text{and}\overrightarrow{B}=-\hat{i}+3\hat{j}-8\hat{k}$  is                     

$\overline{B}=-\hat{i}+3\hat{j}-8\hat{k}$

$\overline{A}+\overline{B}=3\hat{i}+6\hat{j}-2\hat{k}$

Unit vector parallel to $\overline{A}+\overline{B}$  is

$\frac{\overline{A}+\overline{B}}{\left|\overline{A}+\overline{B}\right|}=\frac{3\hat{i}+6\hat{j}-2\hat{k}}{7}=\frac{1}{7}\left(3\hat{i}+6\hat{j}-2\hat{k}\right)$