Answer
$ \frac{5\pm 5i\sqrt {13} }{14}$
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 $i^2=-1$.
If I multiply the equation by $5$ (this doesn't modify the solutions), the coefficients will be $7,-10,50$.
Hence here $x = \frac{ - (-10) \pm \sqrt {(-10)^2 - 4\cdot7\cdot50} }{2\cdot7}= \frac{10\pm \sqrt {100 - 1400} }{14}= \frac{10\pm \sqrt {-1300} }{14}= \frac{10\pm 10i\sqrt {13} }{14}= \frac{5\pm 5i\sqrt {13} }{7}$
