U is a subspace of $\displaystyle R^4$

U = {(a,b,c,d)$\displaystyle \in R^4$| b-2c+d = 0}

Find a basis for U.

Unfortunately, I don't even know where to begin. How do I know how many vectors are needed to span U. If someone can help me out and/or start me off, I'd really appreciate it.

Thanks