How can we show that $U(L)/U(A)$ is free left $U(A)$-module?

Question

Let $U(L)$ and $U(A)$ be the universal enveloping algebras of Lie algebra $L$ and its given subalgebra $A$. We consider $U(L)/U(A)$. It is clear that $U(L)/U(A)$ is a left $U(A)$-module. But I want to prove that it is free left module. May you please let me know the way of showing that?