Question

z^2+8z-14=0

Answer 1

\[z= -4 \pm \sqrt{30}\]

Answer 2

You could complete the square.
z^2+8z-14=0 add 14 to both sides

z^2+8z =14 complete the square
(z+4)^2 = 14+16

(z+4)^2 = 30 square root both sides
z+4 = +-sqrt 30 subtract 4 from both sides

z = -4 +-sqrt 30