Step 1: First we have to write polynomials in descending order.
$$ \begin{aligned} P(x) &= 3x^2+4x-1 \\ Q(x) &= 6x^2-7x+2 \\ \end{aligned} $$Step 2: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrr} & 3x^2 & 4x & -1 \\ \color{red}{ - } & 6x^2 & -7x & 2 \\ \hline \end{array} $$Step 3: Change ALL the signs in the second row.
$$ \begin{array}{lrrr} & 3x^2 & 4x & -1 \\ \color{red}{ - } & -6x^2 & 7x & -2 \\ \hline \end{array} $$Step 4: Combine like terms (add each column):
$$ \begin{array}{lrrr} & 3x^2 & 4x & -1 \\ \color{red}{ - } & -6x^2 & 7x & -2 \\ \hline & 9x^2 & -3x & 1 \end{array} $$