Need c and d section answered with data from $1$st picture

PE $21-1$A$.$ High$-$Low Method OBJ. $1,$ Example Exercise $21-1$ p$.969$ The manufacturing costs of Buckley Industries for three months of the year are provided below. Using the high$-$low method, determine $($a$)$ the variable cost per unit and $($b$)$ the total fixed cost.

Units $=$

$40,000$