Hello,

I'm not an English speaker, so I'm sorry if I don't write very well (especially because it's not "common" language haha)

So, to solve this problem, I don't know if you know "rho" (it's the name of the Greek letter; I've just searched about the name in English, but I don't find, anyway).

Well, you may know, so simply, you have to calculate "rho", which would be 

rho = b² - 4*a*c = 24² - 4*a*6 = 24² -24a = 24 (24 - a)

and you know that there are exactly two (different) roots when "rho" is strictly positive (so not equals to zero).

Then , rho > 0 <=> 24 - a > 0 <=> 24 > a <=> a < 24

Then the answer for this question is a<24 (if I did it right)