Using the notation Vab consistently for the relative velocity of a with respect to b, so that the first subscript stands for the moving object and the second subscript stands for the reference frame or body, relative velocity vectors in different reference frames may be related by vector addition as follows:

     Vac = Vab + Vbc

In Andes you will apply this equation in terms of components, as: 

    Vac_x = Vab_x + Vbc_x
    Vac_y = Vab_y + Vbc_y   

Notice that with this notation,  the subscripts on the sum vector on the left-hand side (a and c) match the outer subscripts on the right-hand side, and the two inner subscripts on the right-hand side of the equation are the same (b).