How did they get -3 for this problem?

Step-by-step explanation:

Solve x^2 + 6x + 4.  Here the coefficients of the quadratic are a = 1, b = 6 and c = 4.  The discriminant is therefore b^2-4ac, or, in this case, 6^2-4(1)(4), or 20.

The roots are:

      -6 ± √20  

x = = -3 ± √4√5:  x = -3 + 2√5 and x = -3 - 2√5

             2

The -3 comes from the -6/2 in the application of the quadratic formula.

Answer A is correct.