The acceleration of a particle moving only on a horizontal xy plane is given by a = 3ti + 4tj, where a is in meters per second-squared and t is in seconds. At t = 0, the position vector r = (20.0m)i + (40.0m)j locates the particle, which then has the velocity vector v = (5.00m/s)i + (2.00m/s)j. At t = 4.00 s, what are

(a) Its position vector in unit-vector notation and

(b) The angle between its direction of travel and the positive direction of the x axis?