Step 1: First we have to write polynomials in descending order.
$$ \begin{aligned} P(x) &= -3x^5+9x^3+5x \\ Q(x) &= 3x^5+8x^3-2x \\ \end{aligned} $$Step 2: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrr} & -3x^5 & 9x^3 & 5x \\ \color{blue}{ + } & 3x^5 & 8x^3 & -2x \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrr} & -3x^5 & 9x^3 & 5x \\ \color{blue}{ + } & 3x^5 & 8x^3 & -2x \\ \hline & & 17x^3 & 3x \end{array} $$