Part A would be 2 lines below the surface because since he is 2 lines above the surface (represented by +10 feet).
Part B would be -50 feet because he would be BELOW the surface (represented by "-", like -45).
Part C would be 20 because DeWayne would be at the tip of the seaweeds, -20 feet. Since each line is 5 feet, then he would have to swim 20 feet up to reach the surface.
Part D would be 0 feet. Since one line above the surface is 5, and one line below the surface is -5, subtracting or adding 5 would be 0.
:)
Hope this helped,
Loafly