Answer
$\frac{ - 4\pm i\sqrt {5} }{2}$
Work Step by Step
The quadratic formula says $x = \frac{ - b \pm \sqrt {b^2 - 4ac} }{2a}$ when $ax^2 + bx + c = 0$.
We know that $i^2=-1$.
Hence here $x = \frac{ - 16 \pm \sqrt {16^2 - 4\cdot4\cdot21} }{2\cdot4}= \frac{ - 16 \pm \sqrt {256 - 336} }{8}= \frac{ - 16 \pm \sqrt {-80} }{8}= \frac{ - 16 \pm 4i\sqrt {5} }{8}= \frac{ - 4\pm i\sqrt {5} }{2}$
