\begin{gathered}
  f(x) = 2x + 3 \hfill \\
  g(x) = 4x - 5 \hfill \\
   \hfill \\
   \hfill \\
  f(a) = 2(a) + 3 \hfill \\
  f(b) = 2(b) + 3 \hfill \\
  f(c) = 2(c) + 3 \hfill \\
  ... \hfill \\
  ... \hfill \\
  ...f(g) = 2(g) + 3 \hfill \\
  tj. \hfill \\
  f(g) = 2(4x - 5) + 3 = 8x - 10 + 3 = 8x - 7 \hfill \\
  odnosno \hfill \\
  f \circ g = f(g) = 8x - 7 \hfill \\
   \hfill \\ 
\end{gathered}