Time complexity problem

Let Σ = {0, 1} and let A ⊆ Σ* be a language contained in DTIME(4n), and define

B = {xx | x ∈ A}.

(a) Show that B ∈ DTIME(2n).

(b) Prove that A ≤pm B.

I’m new to complexity theory. how can I show that if the language has the time complexity O(4n) how can i got about proving that its sublanguage has the complexity O(2n). also any help with the part(b) would also be appreciated.