A submarine starts out at 135 feet below sea level. It dives 239 feet before rising 307 feet. What is the current depth of the submarine?
Since "sea level" has a depth of zero, the submarine starts at a depth of -135 feet.
After diving down an additional 239 feet, its depth is: -135 -239 = -376
Then, after rising 307 feet, its depth becomes: -135 -239 +307 = -69 feet
