It is given in the question that B is in between C and D.
Therefore
[tex]CB+ BD = CD[/tex]
Substituting the given values of CB,BD and CD, we will get
[tex]4x-9+3x+5=17[/tex]
Combining like terms, we will get
[tex]7x-4=17[/tex]
Now we need to move 4 to the right side by adding 4 to both sides, that is
[tex]7x = 21
\\
x=3[/tex]
