The expression f(g(x)) refers to the function composition. That term is defined as follows:
[tex]f\circ g(x)=f(g(x))[/tex]
Let's suppose that x is in the domain of g, then
[tex]g(x)=y[/tex]
And we need to find f(y):
[tex]\begin{gathered} g(x)=3x-9 \\ \Rightarrow f(g(x))=f(y)=2y+3=2(3x-9)+3 \\ \Rightarrow f(g(x))=6x-18+3=6x-15 \\ \Rightarrow f(g(x))=6x-15 \end{gathered}[/tex]
Therefore, the answer to the question is the second option 6x-15
