Ascuba diver dove down from a surface of the ocean to a elevation of -29 7/10 feet at a rate of -18.8 feet per minute. after spending 12.75 minutes at the elevation, the diver ascended to a elevation o -28 9/10 feet. the total time for the dive so far was 19 1/8 minutes. what was the rate of change in the divers elevation during the ascent?