This is one of the most complex problems I've seen on Brainly. If you don't understand after reading my lengthy explanation, just tell me.
You have 5000 feet of reel. The office building uses 3127 feet of cable.
So you do 5000-3127 to find the remaining amount of reel.
5000 - 3127 = 1873
Whew! That was quite difficult, wasn't it?
