someone please help me solve this exercise. i already tried positive and negative 0.006403 and says its wrong.

Check my work mode : This shows what is correct or Incorrect for the work you have completed Light emitting diode (LED) light bulbs have become required in recent years, but do they make financial sense? Suppose a typical 60 watt incandescent light bulb costs $.42 and lasts for 1000 hours. A 15 watt LED, which provides the same light, costs $3.25 and lasts for 12,000 hours. A kilowatt-hour is 1000 watts for 1 hour. Suppose you have a residence with a lot of incandescent bulbs that are used on average 500 hours a year. The average bulb will be about halfway through its life, so it will have 500 hours remaining and you can't tell which bulbs are older or newer). If you require a return of 11 percent, at what cost per kilowatt-hour does it make sense to replace your incandescent bulbs today? (A negative answer should be indicated by a minus sign. Do not round Intermediate calculations and round your answer to 6 decimal places, e.g., 32.161616.) Answer is complete but not entirely correct. Break-even cost s -0.006403