\(x^2-78x+993=0\\ \Leftrightarrow\left\{{}\begin{matrix}x=\dfrac{-\left(-78\right)+\sqrt{\left(-78\right)^2-4\cdot1\cdot993}}{2\cdot1}=39+4\sqrt{33}\\x=\dfrac{-\left(-78\right)-\sqrt{\left(-78\right)^2-4\cdot1\cdot993}}{2\cdot1}=39-4\sqrt{33}\end{matrix}\right.\)

He's twenty years old.