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I've lately noticed that but maybe others who would start this course would search for the answer as I did, so the reply may benefit them

Your code is fine, and also formula is fine.

you only have missed some +/- places
you should set:
+ Math.sqrt(d - 4 * a * c)
- Math.sqrt(d - 4 * a * c)
instead of:
+ Math.sqrt(d - 4 * a * c)...