At time t= O, a particle is located at the point (8,7,8). lt travels in a straight line to the point (1,3,1), has speed 2 at (8,7,8) and constant acceleration - 7i - 4j - 7k. Find an equation for the position vector r(t) of the particle at time t. The equation for the position vector r(t) of the particle at time t is r(t) = ( (Type exact answers, using radicals as needed.)