The velocity is the derivative of the position function. Differentiate with respect to t.
Since x(t) is the position of the particle at time t, then the derivative x′(t) is the velocity of the particle at time t. Furthermore, is then equal to the acceleration of the particle. In this case, it appears the question is asking you to find where the accelaration of this particle is positive. So solve for the second deriv of x(t) and study its sign as t varies between 0 and 3.