Step 1: First we have to write polynomials in descending order.
$$ \begin{aligned} P(x) &= 17x+7 \\ Q(x) &= 19x \\ \end{aligned} $$Step 2: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrr} & 17x & 7 \\ \color{blue}{ + } & 19x & \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrr} & 17x & 7 \\ \color{blue}{ + } & 19x & \\ \hline & 36x & 7 \end{array} $$