A 26-foot tall oil tank began the day completely full.  It is being emptied into a transport truck at a...

A 26-foot tall oil tank began the day completely full.  It is being emptied into a transport truck at a constant rate.  The depth of the oil in the tank is a function of the nummber of minutes the tank has been emptying.  The equation y = 26 - (1/5)x  can be used to determine y, the depth of oil remaining in the tank after x minutes.  Determine the rate of change in this equation.