</h2>Answer mentioned above is correct but there is a also shortcut builtin which is math auto correct. It is much like LaTeX. By default its inactive but you can activate it and is really helpful if you want to write big equations. For eg if you want to type H<sub>2</sub> than you just have to type <b>H_2</b> etc and many more options are available like for superscript character following caret (^) sign will be converted as superscript. Many such shortcuts are covered in this <a href="https://www.youtube.com/playlist?list=PLbTE-xLDPxtBP-TE2fS1MysSqFCkHh1N3"> video </a> or you can simply search for <emph> <b> How to insert mathematical equation (like LaTeX) in Ms-Office: Tips and tricks </b> </emph> on youtube. 
This method will be especially helpful if you are a fast at typing. Moreover it will save your time which is lost while switching between keyboard and mouse and searching proper option in word.</h2>