OpenSAT

To solve for $x$, we can use the quadratic formula. The equation is in the form $ax^2 + bx + c = 0$, where a = 1, b = 4, and c = -21. Substituting these values into the quadratic formula gives: $x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a} = \frac{-4 \pm \sqrt{4^2 - 4(1)(-21)}}{2(1)} = \frac{-4 \pm \sqrt{16 + 84}}{2} = \frac{-4 \pm \sqrt{100}}{2} = \frac{-4 \pm 10}{2}$. Therefore, $x = 3$ or $x = -7$. Of the choices given, $x = 3$ is the correct answer.