Follow these steps to create a piecewise function:

1. type the ` (it's in the upper left corner of the mac keyboard).  You will then see ``  appear with a cursor in the middle.

2. type using {(line 1 value, if statement), (line 2 value, if statement):}

Example below:

x/x = {(1,if x!=0),(text{undefined},if x=0):}

would give you: