How can a line be broken inside $$ in Latex?

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To break a line inside $$ in Latex, you can use the \newline or \ command. For example:

$$y = mx + b \newline z = x + y$$

Or

$$y = mx + b \ z = x + y$$

Both of these commands create a new line within the equation. Note that it is advisable to use \begin{align} and \end{align} instead of $$ for multiline equations as they provide better alignment and spacing options.