SA: You have two lines from C to X and none from C to Y.

for your problem: fill the 3 gallon container twice and dump it into the 7 gallon one, making 6 gallons. Then fill the 3 gallon container once more and dump what you can into the 7, leaving 2 gallons in the 3. Dump out the 7 and pour the 2 gallons into it. Then just fill up the 3 gallon container again and dump it into the 7. 5 gallons.

I don't have time to look up a puzzle right now, so someone else can post one.