Given a vector for acceleration at time t one can find the vector for the velocity at the same time t by integrating with respect to t the vector for acceleration.
if [itex]\underline{a}[/itex] = 4t[itex]\underline{i}[/itex] + t[itex]^{2}[/itex][itex]\underline{j}[/itex]
then [itex]\underline{v}[/itex] = 2t[itex]^{2}[/itex][itex]\underline{i}[/itex]+ [itex]\frac{t^{3}}{3}[/itex][itex]\underline{j}[/itex] + some constant vector.