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The Area between Curves

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The blue line is the graph of $f\left(x\right)=x+3$.
The red line is the graph of $g\left(x\right)=2x$.
Find the area enclosed by $f\left(x\right),g\left(x\right)$ and the $x$-axis (shaded green).
$Area =$
Consider the following two approaches:

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$-$
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$=$
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$\displaystyle \int_0^3{x+3\;dx}=$
$-$
$\displaystyle \int_0^3{2x\;dx}=$
$=$
$\displaystyle \frac{9}{2}$
OR
$\displaystyle \int_0^3{x+3\;dx}-\displaystyle \int_0^3{2x\;dx}=\displaystyle \int_0^3$ $dx$
$=$
$\displaystyle \frac{9}{2}$