Let $f : \mathbb{R}\to \mathbb{R}$ be given.
Suppose that $\lim_{x\to 0} f(x) = L$ and $f(0) = L$ . Prove that $\lim_{x\to 0}(f \circ g)(x) = L$, where $g(x) = xD(x)$:
How do I approach this? Fairly new to Calc and I tried looking for examples for some time but I can't quite explain