How to recreate the following diagram?

Question

I want to recreate the following diagrams

Here is my MWE

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{calc,intersections} 
\begin{document}
\begin{tikzpicture} 
    \draw [->, name path=x] (-1,0) -- (9.5,0) node [right] {$x$};
    \draw [->] (0,-1) -- (0,6) node [above] {$y$}; 
    \node at (0,0) [below left] {$0$}; 
    \coordinate (start) at (1,0);
    \coordinate (k) at (2.6,0) ;
    \coordinate (c) at (4.5,3.5);
    \coordinate (end) at (9,0);
    \draw [thick,name path=curve] (start) to[out=70,in=180]
        (c) to[out=0,in=180] (end);
        \draw [dashed] (2.6,2.81)node [above,left] {$(a,f(a))$} --(k) node [below=1mm] {$a$};
        \draw [dashed] let \p1=(c) in (c) -- (\x1,0) node [below] {$x$};
        \draw (4.5,3.5) node [above] {$(x,f(x))$}; 
\end{tikzpicture}
\end{document}