How can I show that these two lines are perpendicular?

$\displaystyle \b{r} = \left(2\b{i}+1\b{j}+1\b{k}\right)+\lambda \left(-2\b{i}+4\b{j}+2\b{k}\right)$

$\displaystyle \b{s} = \left(5\b{i}-4\b{j}+2\b{k}\right)+\lambda \left(-1\b{i}+1\b{j}+3\b{k}\right)$