The solution of the linear system $$\begin{array}{rcrcrcrcl} r &+& s &+& t &+& u &=& 6 \\ r &+& 2s &+& 3t &+& 4u &=& 28 \\ r &-& s &+& 2t &-& u &=& 6 \\ r &+& 2s &-& 3t &+& 2u &=& -8 \\ \end{array}$$
is $r =$ , $s =$ , $t =$ , $u =$ .