Provide feedback if someone gets the right answer with the wrong graph Fix #33379

Summary: Curious if something like this would be covered by static questions?

Test Plan: Test locally, answering with correct answer and wrong graph, then with a fully correct answer. Check problem history to ensure the timeline worked.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D1066
added exercises about simple implicit differentiation (ax^2+by^2=c)
Merge branch 'master' of git://github.com/Khan/khan-exercises