At the end of a snow storm, Emily saw there was a lot of snow on her front lawn. The temperature increased and the snow began to melt at a steady rate. There was a depth of 15 inches of snow on the lawn when the storm ended and after 4 hours, the depth of snow was down to 12 inches. Write an equation for S, S, in terms of t, t, representing the depth of snow on Emily's lawn, in inches, tt hours after the snow stopped falling.