$2.)$ Over the previous decade, about $13$ billion tickets were sold at US movie theaters. The marginal revenue from box$-$office receipts $($in billions of dollars per billion tickets$)$ was approximated by

$R^{\text{'}}(x)=0\mathrm{.}56x+3\mathrm{.}1$

Where $x$ is the number of tickets sold $($in billions$).$ What was the total revenue from selling these $13$ billion tickets?