# How do you evaluate the function f(x)=6x when x=3?

Substitute $\textcolor{red}{3}$ for $\textcolor{red}{x}$ in $f \left(x\right)$ and calculate the result:
$f \left(\textcolor{red}{x}\right) = 6 \textcolor{red}{x}$ becomes:
$f \left(\textcolor{red}{3}\right) = 6 \times \textcolor{red}{3}$
$f \left(\textcolor{red}{3}\right) = 18$