At the end of a snow storm, Elijah saw there was a lot of snow on his front lawn. The temperature increased and the snow began to melt at a steady rate. There was a depth of 18 inches of snow on the lawn when the storm ended and then it started melting at a rate of 1.5 inches per hour. Write an equation for S, S, in terms of t, t, representing the depth of snow on Elijah's lawn, in inches, tt hours after the snow stopped falling.