Step 1: First we have to write polynomials in descending order.
$$ \begin{aligned} P(x) &= -3x^4+6x^3-2 \\ Q(x) &= 7x^4+2x^3-7x^2 \\ \end{aligned} $$Step 2: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrrr} & -3x^4 & 6x^3 & & -2 \\ \color{red}{ - } & 7x^4 & 2x^3 & -7x^2 & \\ \hline \end{array} $$Step 3: Change ALL the signs in the second row.
$$ \begin{array}{lrrrr} & -3x^4 & 6x^3 & & -2 \\ \color{red}{ - } & -7x^4 & -2x^3 & 7x^2 & \\ \hline \end{array} $$Step 4: Combine like terms (add each column):
$$ \begin{array}{lrrrr} & -3x^4 & 6x^3 & & -2 \\ \color{red}{ - } & -7x^4 & -2x^3 & 7x^2 & \\ \hline & 4x^4 & 8x^3 & -7x^2 & -2 \end{array} $$