\documentclass[12pt,a4paper,fleqn]{report}
\usepackage[utf8]{inputenc}
\usepackage[finnish]{babel}
\usepackage{amsmath}

\usepackage[hmargin=2.0cm,vmargin=2.5cm]{geometry}

\usepackage{setspace}
\setstretch{1.5}

\usepackage{fancyhdr}
\pagestyle{fancy}
\fancyhead[L]{Date}
\fancyhead[R]{Name}
\fancyfoot[C]{}

\renewcommand{\headrulewidth}{0pt}
\renewcommand{\footrulewidth}{0pt}


\begin{document}

\chapter*{big}
\section*{ \huge Header0 }
\section*{ Header1 }


\subsection*{ Header2 }

\subsubsection*{ Header3 }
\[ a+b=c \iff a=c-b \]
See this:
\begin{align*}
a = \frac{b}{c} \\
c = \frac{b}{a} 
\end{align*}



\end{document}