By factoring, find two values j and k which satisfy two conditions, jk=ac=-12 and j+k=b=-4, so j and k must be 2 and -6 so the factors are:
(x+2)(x-6)=0
So x=-2 and 6
By the quadratic formula, x=(-b±√(b^2-4ac))/(2a), in this case:
x=(4±√(16+48))/2
x=(4±√64)/2
x=(4±8)/2
x=2±4
x=-2 and 6