Suppose that B is the midpoint of segment AC; therefore, AB=BC
Thus,
[tex]\begin{gathered} AB=BC \\ \Rightarrow2x+21=5x-9 \end{gathered}[/tex]
Solve for x as shown below
[tex]\begin{gathered} \Rightarrow3x-9=21 \\ \Rightarrow3x=30 \\ \Rightarrow x=10 \end{gathered}[/tex]
Therefore, the answer is x=10
