Having some trouble with a homework problem:

Prove the identity $\displaystyle V = \sum V[x_i]U_i$, where $\displaystyle x_1, x_2, x_3$ are the natural coordinate functions.

The book then gives the hint, evaluate $\displaystyle V = \sum v_i U_i on x_j$

Any help doing this would be appreciated, I'm a bit stuck.