Step 1: First we have to write polynomials in descending order.
$$ \begin{aligned} P(x) &= 6x^2-11x+4 \\ Q(x) &= -3x^3+x^2+10x \\ \end{aligned} $$Step 2: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrrr} & & 6x^2 & -11x & 4 \\ \color{blue}{ + } & -3x^3 & x^2 & 10x & \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrrr} & & 6x^2 & -11x & 4 \\ \color{blue}{ + } & -3x^3 & x^2 & 10x & \\ \hline & -3x^3 & 7x^2 & -x & 4 \end{array} $$