It really just comes with time and practice. The denominator is very easy, assume we have ax^2+bx+c. Right away the denimonator is 2*a, that's it! The numorater is harder, think of it as:
Minus the middle coeficient (-b)
Plus Minus
Sqrt of ((b again :D)^2 - (4*(The other two coefficents not mentioned before in the numerator (a*c)))
Hope that helps!
