Y = 2X_1 - 3X_2 +4X_3
Then highlight it, and press the sigma button in the editor (between the php button and the youtube button)
This will "wrap" math tags around your code, which tells the site to read it as Latex syntax.
It will display like this
You could also just type the tags, which is what I usually do.
I attached a picture to show you what it should look like when done.