.
The candle burns in 0.6*4 = 2.4 centimeters (4 is 4 hours = the time interval between "7 hours" and "11 hours").
Hence, the length of the candle was 23.4 + 2.4 = 25.5 cm.
You can solve the problem using equations.
You also can solve it using your common sense.
It is good if both answers coincide.
----------------
For problems on burning candles see the lesson
- Burning candles
in this site.