Answer
$$
6
$$
Work Step by Step
Using the average value: $=\frac{1}{-1+3} \int_{1}^{3} 3 x d x=\left[\frac{3}{2} x^{2}\right]_{1}^{3}.\frac{1}{2}=\left[-\frac{3}{2} \cdot 1^{2}+\frac{3}{2} \cdot 3^{2}\right]\frac{1}{2}=\left[-\frac{3}{2}+\frac{27}{2}\right]\frac{1}{2}=\frac{24}{2} \cdot \frac{1}{2} =6$
