I would put it like this .
Let x be punch bowl
Ratio: (Sugar/glass)
(3) * (2/8) = x
6/8 = x
0.75= x
So you will need 3/4 cups of sugar
Or if easier for you cross multiply the set up same ratio so would look like .
2/8=x/3. Now do cross multiply
2×3=8×x
6=8x
Now divide
6/8=x
You will get 0.75 which is 3/4 cups