Question 63914
You solve it like this:

Filling  time   21 min
Filling  Rate   1/21 of the tub per minute....A
Draining Rate   1/28 of the tub per minute....B
So, Net Filling rate (NFR) = A-B = (1/21) - (1/28)
Taking LCM of 84, we get NFR = (4/84)-(3/84)=1/84

For filling 1/84 of the tub it takes 1 minute
for filling full tub ie 84/84 it will take 84 minutes.

Good day..
Cross Check : 
In 84 minutes the fill  pipe will pour  4 times the tub volume
In 84 minutes the Drain pipe will drain 3 times the tub volume

We have one volume of water in tub.
Ok!