To answer this question, we will define some things first.
For every mathematical expression or term, it consist of three parts:
1) Coefficient
2) Variable: a symbol that stands in for an unknown value in a mathematical expression
3) Constant
In the expression given:
[tex]\begin{gathered} 9+2z \\ 9\text{ is the constant} \\ 2\text{ is the coefficient} \\ z\text{ is the variable} \end{gathered}[/tex]So the variable in the expression is z.
