Answer:
Part 1
total incremental cost of making 90,000 units =  $388,000
total incremental cost of buying 90,000 units = $396,000
Part 2
There is a cost advantage of $8,000 of making than buying, therefore  the company should continue to manufacture the part.
Explanation:
total incremental cost of making 90,000 units
Variable costs are ($3.20 x 90,000 units)    $288,000
Fixed Costs                                                      $100,000
Total                                                                 $388,000
total incremental cost of buying 90,000 units
Purchase Price ($4.40 x 90,000 units)          $396,000
Total                                                                 $396,000
Decision :
There is a cost advantage of $8,000 of making than buying, therefore  the company should continue to manufacture the part.