Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrr} & 7j^3 & & -2 \\ \color{blue}{ + } & 5j^3 & -j & -3 \\ \hline \end{array} $$Step 2: Combine like terms (add each column):
$$ \begin{array}{lrrr} & 7j^3 & & -2 \\ \color{blue}{ + } & 5j^3 & -j & -3 \\ \hline & 12j^3 & -j & -5 \end{array} $$