Answer
$30$
Work Step by Step
Firstly we need to sketch graph of $f\left( x\right) =8-x;$ and $f\left( x\right) =0$ and show the area between these graphs within given interval
As shown in Graph area is trapezoid
So the area can be calculates as
$A=\dfrac {a+b}{2}\times x=\dfrac {8+2}{2}\times 6=30$
