A single conservative force = (4.0x - 12) N where x is in meters, acts on a particle moving along an x axis. The potential energy U associated with this force is assigned a value of 20 J at x = 0. (a) Write an expression for U as a function of x, with U in joules and x in meters. U(x) = (b) What is the maximum positive potential energy? (c) At what negative value of x is the potential energy equal to zero? (d) At what positive value of x is the potential energy equal to zero?