After this 120-second period , how much time, in seconds will it take for the scientist to travel back to sea level at the submarine's maximum speed of 4.8 feet per second?

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graphically?   well, you graph them both, the solutions is where they both intersect each other, since there are two solutions, meaning they will meet twice, or namely the linear will run into the quadratic curve twice, and where they meet, those x,y coordinates are the solutions to the system.