Let's make a diagram to visualize the problem.
The total momentum of the cars in the Eastern direction would be the addition of each momentum vector. However, there's only one Eastern momentum vector exerted by the first car (1,215 kg), so we just have to find its momentum.
[tex]p=mv[/tex]Where m = 1,215 kg and v = 11.9 m/s.
[tex]p_{\text{east}}=1,215\operatorname{kg}\cdot11.9(\frac{m}{s})=14,458.5\operatorname{kg}\cdot\frac{m}{s}[/tex]
