Given the information, we know that the tank starts holding 18000 gallons of water. Then after 1 day passes, the tank now holds 18000-450=17550 gallons of water. With this, we can find the slope and the equation required:

Now, using y=mx+b, we have the following equation:

Finally, to know how long will it take to reach 6000 gallons, we make y=6000 and solve for x:

Therefore, it will take 26.67 days to reach 6000 gallons of water