This question is all about dimensional analysis. The whole idea behind dimensional analysis is that you multiply a starting value with different conversion factors to cancel out units you don't want to tern the value into the units that you do want. The conversion for yards to feet can be written as 3 feet/yard (this is the conversion factor it can be written as 3feet/1yard or 1yard/3feet, it depends on what you are starting with). If you start out with yards and and want to go to feet you have to multiply the number of yards by 3feet/yard so that the yards cancel out leaving you with feet. If start out with feet and want to go to yards you divide the number of feet by 3feet/yard so that the feet cancel out and you are left with yards.
In your question Tom is trying to go from feet to yards. Therefore he has to divide the number of feet by 3feet/yard to get feet to cancel out. His mistake was that he multiplied by 3feet/yard instead of divide by 3feet/yard. The correct way do it is divide 379 feet by 3feet/yard to get 126.33 yards.
Let me know if anything is unclear to you in the comments. This is a very important skill to learn since this is the basis of many high school and college science classes.
I hope this helps.
