OK, can you also show how you're getting s(3)? Because, if the equation is s(t) = (2/3)t^2 + 4t^2 + 6t + 2, then at t = 3, the current position is 6 + 36 + 18 + 2. Which is equal to 62. If the equation is s(t) = (2/3)t^2 - 4t^2 + 6t + 2, then at t = 3, the current position is 6 - 36 + 18 + 2...