Answer: 42 feet below the sea level
Step-by-step explanation: To solve for the depth, simply just plug in 6 into the equation by substituting "m" since the variable represents minutes, and the submarine has been traveling for 6 minutes.
f(6) = -120 + 13(6)
f(6) = -120 + 78
f(6) = -42
When solved, the answer is 42.