Consider the function f(x) = x4 128x2. Determine the number of points on the graph of y = x) that have a horlzontal tangent line. In other words, determlne the number of solutions to f Determine the values of x at which x) has a horizontal tangent line. Enter your answer as a commaseparated list of values. The order of the values does not matter. Enter DNE if x) does not have any horizontal tangent lines. :i