One possible way: -86+(4*12)=-38 -86 is the number where the diver is. 86 feet below the surface. 4*12 is the total number of feet he has ascended in 12 minutes. Because he is ascending, so you add 4*12 (or 48) to -86. The answer is -38, meaning he's 38 feet below the surface.
neverminding for a few seconds the volume of the cylinder, the percentage in the height used by the water will be the same as the percentage of its volume. We know the total amount or 100% of the height is 14, so what is 11 in percentage?