Consider the function f(x) = :2118' Determine the number of points on the graph of y = x) that have a horizontal tangent line. In other words, determine the number of solutions to f'(x) = 0. Determine the values ofx at which f(x) has a horizontal tangent line. Enter your answer as a comma-separated list of values. The order of the values does not matter. Enter DNE if f(x) does not have any horizontal tangent lines