\\documentclass[11pt]{article}
[] brackets are for optional arguments.
\\documentclass[11pt]{article}
# soft return
\\begin{document}
Hello! This is my first \\LaTeX\\ document.\\\\
A rectangle has side lengths of {x+1} and {x+3}.
\\end{document}
# hard return
\\begin{document}
Hello! This is my first \\LaTeX\\ document.
A rectangle has side lengths of {x+1} and {x+3}.
\\end{document}
The equation ${A(x) = x^2 + 4x + 3}$ gives the area of the rectangle.
Curly brace keeps that inline part to be placed entirely in one line instead of being wrapped at the end.

# inline
$ ... $
# displayed
$$ ... $$
superscripts $$2x^34$$

In order to fix:
superscripts $$2x^{34}$$
