Write an equation in Latex

(1) Basic

\begin{align}
    y = f(x) \\ %automatically numbering, "\\" means "change the line"
    y = g(x) \nonumber \\ % "\nonumber" means "do not numbering this equation"
    y = \frac{1}{x} \nonumber
\end{align}

(2) Alignment

\begin{align}
    y &= f(x) \nonumber \\
    &= T_{0}+ (T_{0}-T_{h})e^{-x} % "&"s are automatically aligned them among themselves.
\end{align}