Step 1: First we have to write polynomials in descending order.

$$ \begin{aligned} P(x) &= 7x^2-4 \\ Q(x) &= -4x^4+6x^3-4x \\ \end{aligned} $$

Step 2: Write second polynomial below the first one. Making sure like terms are lined up.

$$ \begin{array}{lrrrrr} & & & 7x^2 & & -4 \\ \color{blue}{ + } & -4x^4 & 6x^3 & & -4x & \\ \hline \end{array} $$

Step 3: Combine like terms (add each column):

$$ \begin{array}{lrrrrr} & & & 7x^2 & & -4 \\ \color{blue}{ + } & -4x^4 & 6x^3 & & -4x & \\ \hline & -4x^4 & 6x^3 & 7x^2 & -4x & -4 \end{array} $$

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