Question

what is the value of 3f(x) when x is 6

Answer 1

It depends of wich function you are talking about:
For example, if $f(x)=x^2$, just put $x=6$ into formula:

$f(6)=6^2=36$

And multiply by 3:

$3f(6)=108$

So, the answer is $3f(6)$.