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