How can the composite function be evaluated?

Let f(x) and g(x) be two functions. The composite function ${\displaystyle f\circ g}$ can be calculated using the following steps:
1. Evaluate the value of inside function at the given point.
2. Then, take the resultant output as the input to outside function.
Here, $f\circ g=f\circ g(x)=f(g(x))$
Let us assume that the value of function g at point x is $g(x)=t$ where t is some constant
So, $f(g(x))=f(t)$
Let us assume that the value of f at point t is $f(t)=u$, where u is some constant. In this way, composite function can by evaluated.