Let x = your number.
5x = x^2 - 14. To figure out the number, we have to first subtract 5x from each side.
0 = x^2 - 5x - 14. Factor.
0 = (x - 7) (x + 2). Set both terms to 0.
(x - 7) = 0. Add 7 to each side.
x = 7. You're looking for the negative value so this isnt the right one.
Set the other term (x + 2) equal to 0
(x + 2) = 0. Subtract 2 from each side.
x = -2
The negative solution for your number is -2