Suppose you meant (5/6)x = -9/10. We have:
(5/6)x = -9/10
5x = 6(-9/10)
5x = -27 / 5
x = (-27 / 5) / 5
x = -27 / 25
Suppose you meant 5 / (6x) = -9/10. We have:
6x = 5 / (-9 / 10)
6x = (5 * 10) / (-9)
6x = 50 / -9
6x = -50 / 9
x = (-50 / 9) / 6
x = (-25 / 27)