A root of the equation $x^2 - x - 13 = 0$ is —

• a$\dfrac{-1 + \sqrt{51}}{2}$
• b$\dfrac{-1 - \sqrt{51}}{2}$
• c$\dfrac{1 + \sqrt{-51}}{2}$
• d$\dfrac{1 + \sqrt{53}}{2}$