Substituting angular momentum equation to the equation of motion

Question

I'm having a equation $\dfrac{d}{dt}=\dfrac{l}{mr^2}\dfrac{d}{d\varphi}$ which I'm supposed to substitute into $m\ddot{r}-\dfrac{l^{2}}{m r^{3}}=f(r)$ and have a differential equation $\dfrac{1}{r^2}\dfrac{d}{d\varphi}\left(\dfrac{1}{mr^2}\dfrac{dr}{d\varphi}\right)-\dfrac{l^2}{mr^3}=f(r)$ but somehow I just have a huge problem with it. Hope you can help me!