This article covers the Legacy editor. Lessons built in the New editor work differently. To write maths in the New editor, see Writing maths in a lesson. New to the difference between the two? Start with The new lesson experience.
Writing maths
Open the lesson from My Library. The Legacy editor shows the lesson as a list of blocks, with the Outline down the left.
The formatting toolbar has two maths buttons: one for an equation inside a line of writing, and one for an equation on its own line. Both open an editor where you type LaTeX, with a symbol picker for anything you cannot remember.
Click Save when you are done.