Answer: I gave you two answers if you can try them both :)
Explanation:
If his weight on Earth is 126lb and only 21lb on moon, you can divide to see what is the ratio of those weights.
It means that your weight on moon will be 6 times less than on Earth.
Now we have to multiply 31lb which is weight of the person on moon by 6 to get his weight on Earth
On moon our mass becomes 1/6 of actual mass so if you weigh 60 kg then your mass on moon will be 10 kg..
Similarly if your mass on moon is 31 lbs then your mass on earth will be 31*6=186 lbs.