Hi there!
Recall the conservation of momentum for an inelastic collision:
[tex]\large\boxed{m_1v_1 + m_2v_2 = (m_1 + m_2)v_f}}[/tex]
Remember, velocity is a VECTOR and direction must be accounted for. Let the 1000 kg car have a positive velocity and the 1500 kg car have a negative velocity (opposite direction).
Plug in the given values:
[tex](1000)(20) + (1500)(-10) = (1000 + 1500)v_f}}[/tex]
Solve:
[tex]20000 -15000 = 2500v_f}}\\\\5000 = 2500v_f\\\\v_f =\boxed{ 2 m/s}[/tex]