We have some function f(x) and want to evaluate the function for some value of x, in this case for x=8.
Evaluate a function means replace the x for the value you want to evaluate, in this case for 8, so:
[tex]\begin{gathered} f(x)=3(1-x) \\ f(8)=f(x=8)=3(1-8) \\ f(8)=3\cdot(-7)=-21 \end{gathered}[/tex]
