CFG for a given languague


Give a CFG for the languague L = $ \{ 1^n +1^m = 1^{n+m}| n,m \in N_{0}\} $ , with the alphabet Sigma ={1,+,=}.

I am currently trying to solve the given task, I thought a good way is to split the Languague into two more simple languagues, but I am keep failing. May some can help