Make function in function

I want to define a approximated function g(x) from f(x) with neural network but g(x) is too complicated so i want to simplify it to multiple function.

For example: define f = 5x+3 from f=g+3, g=3x.

Answer 1

[Stephen23]

syms x
g = 5*x

g = 

f = g+3

f =