Khan/khan-exercises — pull request #156705 by petercollingridge (2014-01-06 16:23:20+00:00)
Title: Add domain requirement to simplifying rational expressions 4 when needed

In reference to this issue: https://github.com/Khan/khan-exercises/pull/117850
When we divide by x, then there is an additional answer box that requires students to note that z != 0.
Possibly not the most elegant way to do it, but it seems to work.