Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrrr} & 3x^4 & & -5x & 2 \\ \color{red}{ - } & & 4x^2 & -6x & -2 \\ \hline \end{array} $$Step 2: Change ALL the signs in the second row.
$$ \begin{array}{lrrrr} & 3x^4 & & -5x & 2 \\ \color{red}{ - } & & -4x^2 & 6x & 2 \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrrr} & 3x^4 & & -5x & 2 \\ \color{red}{ - } & & -4x^2 & 6x & 2 \\ \hline & 3x^4 & 4x^2 & -11x & \end{array} $$