We have the function cost:
[tex]C(x)=V\cdot x+R[/tex]The renting cost R is $800.
The variable cost V is $2 per unit.
The total cost is $2300.
We can calculate the number of units x as:
[tex]\begin{gathered} C(x)=V\cdot x+R \\ 2300=2\cdot x+800 \\ 2300-800=2x \\ 1500=2x \\ x=\frac{1500}{2} \\ x=750 \end{gathered}[/tex]Answer: the number of units is 750 (x=750).