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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
checked author's solution. it will return 4. not an issue.
That test doesn't exist in either the sample or full test suites. There is a similar one
-5x^2+10x+4
, for which the gradient is indeed -20; I think you've modified the tests and are now failing them because you modified them. Probably not a kata issue.