At the end of a snow storm, Tristan saw there was a lot of snow on his front lawn. The temperature increased and the snow began to melt at a steady rate. After the storm, the snow started melting at a rate of 0.75 inches per hour and it is known that 4 hours after the storm ended, the depth of snow was down to 9 inches. Write an equation for S, S, in terms of t, t, representing the depth of snow on Tristan's lawn, in inches, tt hours after the snow stopped falling.