Let $\displaystyle S$ be the subspace of $\displaystyle R^{4}$ spanned by $\displaystyle x_1=(1, 0, -2, 1)^{T}$ and $\displaystyle x_2=(0, 1, 3, -2)^{T}$. Find a basis for $\displaystyle S^{\bot}$.

I was thinking about multiplying the vectors by a set of vectors $\displaystyle [x_1,x_2,x_3,x_4]^T$ $\displaystyle \epsilon$ $\displaystyle S^{\bot}$ but then since there are two vectors I am lost on where to begin. Help would be appreciated!