How do I find the minimum marginal cost of C(x) = x^3 - 6x^2 +13x + 15 ?

When I take the derivative of C(x), I get 3x^2 - 12x + 13, and when I set that equal to 0, I cannot factor that or use the quadratic formula on it. However, my textbook says the answer is $1. Am I overlooking something?