Step 1: First we have to write polynomials in descending order.
$$ \begin{aligned} P(x) &= -2x^3+x \\ Q(x) &= -7x^3+7x-3 \\ \end{aligned} $$Step 2: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrr} & -2x^3 & x & \\ \color{red}{ - } & -7x^3 & 7x & -3 \\ \hline \end{array} $$Step 3: Change ALL the signs in the second row.
$$ \begin{array}{lrrr} & -2x^3 & x & \\ \color{red}{ - } & 7x^3 & -7x & 3 \\ \hline \end{array} $$Step 4: Combine like terms (add each column):
$$ \begin{array}{lrrr} & -2x^3 & x & \\ \color{red}{ - } & 7x^3 & -7x & 3 \\ \hline & -9x^3 & 8x & -3 \end{array} $$