How do you know what to do first when solving composite function?

So when you're solving a composite function you carry out the function closest to the 'x' first. It is useful to think of a composite function as the function of a function. Let me demonstrate with an example:E.g. f(x)=10x+7 g(x)=3xfind: fg(x)fg(x) could also be written as f(g(x)) so it is the 'f' of g(x)In order to solve this you need to put g(x) into f(x):fg(x) = 10(3x) + 7 = 30x + 7