Final velocity when a 2.0 kg cart at 3.0 m/s sticks to a 1.0 kg cart at rest

Assumptions: the carts stick together, external horizontal forces are negligible, and the motion is one-dimensional.
Conserve momentum: m_1 v_1 + m_2 v_2 = (m_1 + m_2)v_f.
(2.0 kg)(3.0 m/s) + (1.0 kg)(0 m/s) = (2.0 kg + 1.0 kg)v_f
6.0 kg*m/s = 3.0 kg * v_f
v_f = (6.0 kg*m/s)/(3.0 kg)
v_f = 2.0 m/s
The stuck carts move at 2.0 m/s in the original direction of the 2.0 kg cart.