I need help with this proof. it's kinda hard?

If a_n is a sequence with a subsequence b_n, then (infinity)Σ(n=1) b_n is a subseries of (infinity)Σ(n=1) a_n. Prove that if every subseries of (infinity)Σ(n=1) a_n converges, then (infinity)Σ(n=1) a_n converges absolutely