Answer
$\dfrac{9}{2} \ sq. units$
Work Step by Step
We are given that $f(x)=\int_3^6 (x-3) \ dx$
Here, the area of a triangle is equal to $A=\int_3^6 (x-3) \ dx$
$=\dfrac{1}{2} \times (Base) \times Height$
$=\dfrac{1}{2} (6-3)(1)$
Thus, $\int_3^6 (x-3) \ dx=\dfrac{9}{2} \ sq. units$
