The cost of manufacturing [tex] x [/tex] units is
[tex] C=ax+b [/tex], where [tex] a [/tex] cost per unit and [tex] b [/tex] fixed cost.
Given that
[tex] 3500a+b=84400\\
1100a+b=46000 [/tex]
Subtract the second equation from the second,
[tex] (3500-1100)a=84400-46000\\
a=16 [/tex]
The variable cost per unit is [tex] \$16 [/tex].