Please teach me how to get 7.22 step by step and no excel.

We are evaluating a project that costs $252,000, has a four-year life, and has no salvage value. Assume that depreciation is straight-line to zero over the life of the project. Sales are projected at 7,400 units per year. Price per unit is $59, variable cost per unit is $13, and fixed costs are $214,000 per year. The tax rate is 21 percent. Ignore the effect of taxes. We require a return of 10 percent on this project. What is the DOL at an output level of 5,400 units? 7.22 5.34 5.78 6.35