Hi i will give you explanation and examples, if I deserve a brilliant give me thank you :)
f(x)+g(x) Add the two functions g and f
(3x+2)+(4x)
f(x)+g(x)=7x+2
f(g(x)) Put the g function into f
f(4x)
f(4x)=3(4x)+2
12x+2
You could combine functions when you need to find the value of something for two equations. Like finding the total value of tickets sold for adult (f(x)) tickets and children (g(x)) tickets.