Castle Windows is a small company that installs windows. Its cost structure is as follows: Selling price from each window installation Variable cost of each window installation $ 800 600 Annual fixed costs $ 175,000 Use (a) the equation method and (b) the contribution method to calculate operating income if Castleinstalls 2,400windows. Use (a) the Equation method to calculate operating income if Castle installs 2,400 windows. Begin by determining the formula to calculate the operating income using the equation method. Then, calculate the operating income. (Abbreviation used: FC Fixed costs, SP = Selling price, VCU = Variable cost per unit, Q = Quantity of units sold.) )-( )-( =Operating income