If an object moves along a line so that it is at y = f(x) = 4.1:2 2x at time x (in seconds), nd the instantaneous velocity function \"u = fix) and nd the velocity at times x = I, 3, and 5 seconds (y is measured in feet} Problems 65-72 require the use of a graphing calculator. For each problem, find f'(x ) and approximate (to four decimal places) the value(s) of x where the graph of f has a horizontal tangent line. 65. f(x) =x - 3x - 4Vx