Let $T: \mathbb{R}^{3} \rightarrow \mathbb{R}^{2}$ and $S: \mathbb{R}^{2} \rightarrow \mathbb{R}^{3}$ be the linear transformations given by
$$
T\left(x_{1}, x_{2}, x_{3}\right)=\left(x_{1}+x_{2}, x_{2}+x_{3}\right)
$$
and
$$
S\left(x_{1}, x_{2}\right)=\left(x_{1}+x_{2}, x_{1}-x_{2}, x_{2}\right) .
$$
Give similar formulae for $S T$ and $T S$.