Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrr} & 6x^3 & 5x^2 & 4 \\ \color{blue}{ + } & & 4x^2 & \\ \hline \end{array} $$Step 2: Combine like terms (add each column):
$$ \begin{array}{lrrr} & 6x^3 & 5x^2 & 4 \\ \color{blue}{ + } & & 4x^2 & \\ \hline & 6x^3 & 9x^2 & 4 \end{array} $$