Whatever is inside parentheses first [g(x)], is what you would evaluate first, then after that, you would substitute that answer into the f(x) function for EVERY "X" YOU SEE:
It's true. A line segment has a finite length. A line is infinitely long. It takes a lot of something that is finite to cover something that is infinite.
________ Suppose this is a part of a line. Just a part mind you.
.................... these are line segments. How many segments will it take to cover a line if you keep on extending the line to the line's "full" length?