The acceleration of slider A is defined by the relation a= ??2k??k2 ?? v2, where a and v are expressed in ft/s2 and ft/s, respectively, and k is a constant. The system starts at time t = 0 with x = 1.5 ft and v = 0. Knowing that x = 1.2 ft when t = 0.2 s, determine the value of k.