How does the Boston Tea Party show that American colonists supported the following statement?it is in separably essential to the freedom of...

This statement is saying that only American colonial
assemblies (elected by people in the colonies themselves) can tax Americans.  This is
because American colonists were not represented in the British Parliament.  The Boston
Tea Party shows support for this because it was a protest against
taxation.

The Tea Party was a protest against a tax on tea
that was imposed by the British Parliament.  Even though it would have reduced the price
of tea, it was resented because it was imposed by the British Parliament rather than by
an American assembly.  Because of this, the Tea Party (like this statement you give) is
an example of the idea of "no taxation without representation."