At the end of a snow storm, Savannah saw there was a lot of snow on her front lawn. The temperature increased and the snow began to melt at a steady rate. The depth of snow on Savannah's lawn, in inches, can be modeled by the equation S = -1.5t + 15, where t is the time, in hours, after the snow stopped falling. What is the slope of the equation and what is its interpretation in the context of the problem?