Hi, I need some help with the question below!

2. (a) (3 points) Suppose that U and V are both subspaces of R". Prove that U + V = {u+ v: ue U and v E V} is also a subspace of R". (b) (3 points) Write down the subspace U + V explicitly if U = {(1, 21, 31) : 1 E R) and V = {(1, -21, 0) : 1 e R)