Step-by-step explanation:

So, we know both faucets open, can do the whole job in 6minutes
ok, how much has the faster cold water faucet done in 1minute then?
well, let's say the the rate of speed of the cold water faucet is "c", so in 1minute it has done 1/c of the whole work
now, we know the hot water faucet is slower, it takes it 3 times as long, that means is 1/3 the speed of the cold water faucet, so, how much does the hot water faucet do in 1min? well, we know is 1/3 fast as the cold one, the cold one is 1/c, what's 1/3 of 1/c? well, 1/3 * 1/c
now, we know they both added together, can do the whole job in 6 minutes, so in 1minute, they've done 1/6 of the job then
thus

The domain is the set of all point where you can evaluate the function.
As you can see, the first piece is defined from -infinity to 2 (included).
The second piece is defined from 2 to 6 (excluded).
The third piece is defined from 6 (excluded) to +infinity.
So, the domain of the function is composed by all the real numbers which are not 6:

As for the range, it is the set of all values taken by the function. We can see that the first piece spans the y axis from 0 to infinity.
The second piece is constantly equal to 2, so it adds nothing.
The third piece spans the y axis from -2 to -infinity, so the range is

First convert the mixed numbers into fractions:
2 1/6 = 13/6
4 2/3 = 14/3
Now add the fractions:
13/6 + 14/3 = (13 + 14*2)/6 = (13+28)/6 = 41/6
Now convert it back to mixed number
41/6 = 6 5/6
thus 2 1/6 + 4 2/3 = 6 5/6
Hope it helps!