Step 1: First we have to write polynomials in descending order.
$$ \begin{aligned} P(x) &= 7x^3+3x^2+5 \\ Q(x) &= 6x^2+5x-12 \\ \end{aligned} $$Step 2: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrrr} & 7x^3 & 3x^2 & & 5 \\ \color{blue}{ + } & & 6x^2 & 5x & -12 \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrrr} & 7x^3 & 3x^2 & & 5 \\ \color{blue}{ + } & & 6x^2 & 5x & -12 \\ \hline & 7x^3 & 9x^2 & 5x & -7 \end{array} $$