Please help, $(V{)}_i$ does not equal $\frac{?}{\text{b}\text{a}\text{r}}(V{)}_i$ for any species $i.$

As a chief engineer, you are told by one of your staff that the partial molar volume of species A

in a binary mixture of A and $B$ at constant $T,P$ can be modeled as:

$/bar(V{)}_A=21-2(1-x_A{)}^2\frac{c{m}^3}{\text{m}\text{o}\text{l}}$

a$)$ Based on this model, formulate $/bar(V{)}_B$ as a function of $x_B,$ if $V_B=18\frac{c{m}^3}{\text{m}\text{o}\text{l}}.$

b$)$ Determine $/bar(V{)}_A^{}$ and $/bar(V{)}_B^{}.$