Step 1: First we have to write polynomials in descending order.
$$ \begin{aligned} P(x) &= 3x^4-8x \\ Q(x) &= -4x^4+2x \\ \end{aligned} $$Step 2: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrr} & 3x^4 & -8x \\ \color{blue}{ + } & -4x^4 & 2x \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrr} & 3x^4 & -8x \\ \color{blue}{ + } & -4x^4 & 2x \\ \hline & -x^4 & -6x \end{array} $$