Answer
\[ = 18\]
Work Step by Step
\[\begin{gathered}
\int_1^4 {3f\,\left( x \right)} \,dx \hfill \\
\hfill \\
rewrite \hfill \\
\hfill \\
= 3\int_1^4 {f\,\left( x \right)} \,dx \hfill \\
\hfill \\
where\,\,\int_1^4 {f\,\left( x \right)} \,dx = 6 \hfill \\
\hfill \\
then \hfill \\
\hfill \\
= 3\,\left( 6 \right) \hfill \\
\hfill \\
Simplify \hfill \\
\hfill \\
= 18 \hfill \\
\end{gathered} \]
