t1 - 3000 = t - 3000 * 6
t1 = t * 6

My point being, you don't know how many gallons the tanks hold, whether they were empty or full

I'd just say 3000*6 in one, and 3000 in the other, or
3000 in one, and 3000 / 6 in one

there could be many answers.