When you see something like or , you have function composition.
The idea behind function composition is that we start with two separate functions, like and . Then, one function acts as the input for another function.
So, see how has input ? When we have a composition like , the input becomes . This means we need to replace every with .
It's like how cute Sonic replaced creepy Sonic in every scene of the movie after the trailer dropped.
Similarly, we need to replace every with . Try it out! Enter in two functions and see what the composition looks like: