Dear Megan, you're right. Thank you for your comment.
actually though your close its just not quite the right answer. Since its a head on collision one of cars would need to have a negative moment. so m1Vi1+m2Vi2=(m1+m2)Vf 2200(40)+1800(-20)=2200+1800Vf 88000-36000=4000Vf 52000/4000=Vf Vf=13km/hr