Suppose you observe a distant galaxy with a lookback time of 8 billion years

Suppose you observe a distant galaxy with a lookback time of 8 billion years.

What was the maximum possible age for that galaxy when the light we are now observing from it began its journey to Earth? (Hint: Assume the galaxy was born less than a billion years after the Big Bang.)

I did 13.7 - 8 = 5.7 but it says not quite? Some help.

Assuming the galaxy was born less than a billion years after the Big Bang, it can be no less than 4.7 billion years old, and no greater than 5.7.