dy/dx has four factors- , , , and . The first three aresquaredso are never negative- the sign of the derivative is the sign of which changes sign at , the -1.65 and 3.65 you give. But that means that dy/dx> 0 for x> 3.65 or x< -1.65 and is negative for x between -1.65 and 3.65. It is certainly NOT true that the derivative is negative for all x< 0.