The usual discriminant is d=b²-4ac
d = (12)² - 4(3)(5) = 144 - 60 = 84
Answer: 84
When we have an even b like this we can get 1/4 of the usual discriminant with (b/2)²-ac = 6² - (3)(5) = 36 - 15 = 21. That's a bit easier to calculate and usually just as good.