The total cost of producing x units for which the fixed cost are $2500 and the cost per unit $20
Total Cost = Cost per Unit * Units + Fixed Cost
Total Cost = 20x + 2500

The total cost of producing x units for which the fixed costs are $2900 and the cost per unit is $25
Set up the cost function:
Cost function = Fixed Cost + Variable Cost per Unit * Number of Units
Plug in Fixed Cost = 2900 and Cost per Unit = $25
C(x) = 2900 + 25x