You can use the markdown cell to do this. To create a markdown cell, you need to select a cell and then press the escape key followed by the M key.
To check if the cell has been converted to markdown, look for In [ ]: seen to the right of the default code cell. It must be gone once the conversion has taken place. After that, input the following code that uses latex with markdown to represent sub-script/super-script:
Latex subscript:
$x_{2}$
Latex superscript:
$x^{2}$