A general linear equation is

where y is the dependent variable, m the slope or the change per unit, x the independent variable and b the fixed value
then we can identify each one
the dependent variable or the Outcome is P(total pay)
the change per unit is the price per each copy then (70$)
the independent variable is the number of copies N
the fixed values is the base salary 2300
then

no to find the total pay to 24 copies we replace N=24

total pay if Juan sells 24 copies is $3980