Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrr} & 5x & 8 \\ \color{blue}{ + } & x & -1 \\ \hline \end{array} $$Step 2: Combine like terms (add each column):
$$ \begin{array}{lrr} & 5x & 8 \\ \color{blue}{ + } & x & -1 \\ \hline & 6x & 7 \end{array} $$