There are a few ways, but here is one. Start the 5 and 7 minute hourglasses. When the 5-glass finishes there is 2 minutes left in the 7-glass. Flip both and now start measuring the time. First measure the 2 minutes remaining in the 7-glass and when finished flip it two more times to measure the remaining 14 minutes. Flipping the 5-glass is irrelavant for the time measurement at this point.