It's rational because of the repeating digits.
We have
x = 2.732343434…
Then
1,000x = 2,732.343434…
and
100,000x = 273,234.343434…
Subtracting these makes the fractional part go away, leaving you with
100,000x - 1,000x = 273,234.343434… - 2,732.343434…
100,000x - 1,000x = 273,234 - 2,732
99,000x = 270,502
x = 270,502/99,000 = 135,251/49,500
which is clearly a rational number.