Step 1: First we have to write polynomials in descending order.
$$ \begin{aligned} P(x) &= 4x^3-x^2 \\ Q(x) &= x^3+5x^2 \\ \end{aligned} $$Step 2: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrr} & 4x^3 & -x^2 \\ \color{blue}{ + } & x^3 & 5x^2 \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrr} & 4x^3 & -x^2 \\ \color{blue}{ + } & x^3 & 5x^2 \\ \hline & 5x^3 & 4x^2 \end{array} $$