Vectors

The line $l_1$ has equation $\mathbf{r}=\mathbf{i}+\mathbf{j}+\lambda(2\mathbf{i}-\mathbf{j}+\mathbf{k})$. The line $l_2$ has equation $\mathbf{r}=3\mathbf{i}+2\mathbf{k}+\mu(\mathbf{i}+\mathbf{j}-\mathbf{k})$. Find the direction of the common perpendicular to $l_1$ and $l_2$.