It depend on the repeating part.
If x = 0.393939… (the repeating part is underlined), then 100x = 39.393939…, and
100x - x = 39.393939… - 0.393939…
99x = 39
x = 39/99 = 13/33
On the other hand, if you mean x = 0.3999…, then 10x = 3.999… and 100x = 39.999…, so that
100x - 10x = 39.999… - 3.999…
90x = 36
x = 36/90 = 2/5
