*Please show me how to solve this step by step!* Thank you!

8. Swifty Corporation reported the following results from the sale of 4200 hammers in May: sales $168000, variable costs $100800, fixed costs $60000, and net income $7200. Assume that Swifty increases the selling price of hammers by 10% on June 1. How many hammers will have to be sold in June to maintain the same level of net income?

4200

3360

3900

3700